Sects and Islam
While the majority of Muslims share the same fundamental beliefs, with 1.7 billion adherents –close to a quarter of planet Earth's population distributed over the far stretches of the continents and among 49 countries where Muslims are a majority and a history of more than a thousand years, all people who call themselves Muslims are not exactly the same. There are sometimes significant religious differences between them.
Does the existence of sects mean that Islamic teachings (contained in Quran and Sunnah) mandate them? The answer is no. What is important is to realize that unlike other religions, Allah has taken upon Himself to protect Islam - the last revelation and the most complete religion of God to humanity. You will not find any promise from God in the Bible or almost any other religious text that He will protect it. On the other hand, you do find two important promises from God in the Quran:
"God has completed His religion of Islam" (Quran 5:3) God will protect and safeguard His religion form change (Quran 15:9)
All Sects are Not Equal: An Analogy of Circles
An analogy can better explain the issue of sects. The Quran and Sunnah sit at the center around which there are many circles. Some Muslims will lie within a circle, but others will lie outside of it. In other words, some Muslims will be closer to the center, others may be distant. Then, think of a red circle which is so far from the center that anyone who lies outside of that red circle is not even considered a Muslim. The radius of the circle is a measure of how "deviant" the sect is.
In other words, the most "deviant" sects, one can call them cults, will be outside of the red circle. They will be the ones that have the most serious conflict with the established Islamic beliefs and practices. Examples would be Ahmadis, Bahais, and the Druze.
Sects Are Confusing, Who Do I Follow?
What becomes confusing is that with the existence of different sects, who does a new Muslim follow, especially when most sects claim to follow the Quran and Sunnah. What is a new Muslim to do? How does he or she determine who is right and who is wrong? In simple words, how does a new Muslim avoid confusion? Let us break down the answer to this question in a few points:
First, if we go back to the Quran and Sunnah, we will find the answer to this question. You see, the Quran and Sunnah are texts; some will try to take the Quran alone, separating it from the Sunnah (Prophetic traditions). They would then interpret the Quran they way they like. The only way to properly understand the Quran is by going back to the Prophetic traditions and to understand both in light of the understanding of the people who were present at the time of revelation. These texts were revealed in their time, many of the texts were addressed to them, and they had the best teacher (the Prophet of Allah) to explain to them anything that needed explanation. Let us see what the Prophet said regarding this matter,
"The best of people is my generation, then those who come after them, then those who come after them." (Saheeh Al-Bukhari, Saheeh Muslim)
The Prophet distinguished the first three generation of Muslims to be the "best." If they are the "best" according to the Prophet of Islam, then it makes sense that we understand and practice Islam the way the "best" people understood and practiced Islam.
Second, it is important to know that the first generation of Islam is known as the generation of the "Sahabah." The word "sahabah" means "companions" in Arabic. Its singular is "sahabi," which means a "companion." The second and third generation also have names, but "sahabah" is the most important term to know for now.
Third, it is important that one not judge for himself who is Muslim and who is not. When one lacks the proper knowledge, issues such as this should be left to the scholars. There are some sects that do have good and bad incorporated into their beliefs. Take Sufis[as an example. All their beliefs and practices are not wrong, but some are. One should be cautious due to one's lack of knowledge so as to not get confused.
Signs of Deviant Sects:
While there is no easy way to tell who belongs to what sect, the following are some guidelines that warrant caution:
1. Disregard for proofs and evidences based on the Quran and Sunnah.
2. Speaking badly about the sahabah (companions) of the Prophet.
3. Following personal desires and putting them ahead of the Quran and Sunnah.
4. Not paying attention to Islamic Monotheism and hating those who do.
5. Creating division among the Muslims.
6. Rejecting the teachings (Sunnah) of the Prophet Muhammad, claiming the Quran is sufficient.
7. Putting another person (usually the leader of the sect) at the same level of Prophet Muhammad in terms of love, respect, and obedience.
8. Hating the scholars of Islam.
Examples of Cults / Sects Influential in the West
As mentioned earlier, not all sects are equal. Even within a sect, there are sub sects that vary greatly in their teachings. Keeping this in mind, below is a brief discussion of some sects:
1. Ahmadis / Qadiyanis 
Ahmadis or Qadiyanis are a missionary-oriented sect of Indian origin, founded by The Apostate Mirza Ghulam Ahmad (1839-1908) who claimed to be a prophet. The Qadiyanis currently have a presence in many countries, including most Western countries. Their world wide numbers are estimated as high as 10 million. Even though their headquarters are in Pakistan, they have a strong presence in London, UK.
The Qadiyanis also known as Ahmadis and Mirzais, have been declared as non-Muslims by thousands of Muslim scholars. The following statement was issued by the Islamic Fiqh Council:
The claim that Mirza Ghulam Ahmed was a Prophet and that he received revelation makes him and anyone who agrees with him an apostate, who has left the folds of Islam. As for the Lahoris, (Lahore Ahmadiyya Movement for the Propagation of Islam) they are like the mainstream Qadiyanis. The same ruling of apostasy applies to them as well, in spite of their claim that Mirza was a 'shadow and manifestation of Prophet Muhammad'.[Majma' al-Fiqh al-Islami, p. 13]
This was also mentioned in the World Muslim League Conference, held in Makkah, Saudi Arabia, from the 14th to the 18th of Rabi al-Awwal 1394H (April 1974) wherein the members unanimously reached the conclusion that the Ahmadi/Qadiyanis are not Muslims.[http://alhafeez.org/rashid/rabita.html]
Also known as "Sevener Shi'ites." The Ismailis reject the Quran and all forms of prayers found in the main Sunni Islamic tradition. This frees them from obligations such as prayer, fasting, and hajj. They are mostly located in Pakistan, North-west India and the Chinese province of Sin-Kiang. The Khojas, a sub-sect, are mainly to be found in Gujarat, India. There are also Khoja communities in East and South Africa. They are also found in the Western countries. Most Ismaili businesses put the picture of Prince Karim Agha Khan, their leader, at a prominent place in their shop.
3. Bahais 
Bahais follow the teaching of Bahaullah ('splendor of God') (1817-1892). They attract followers by speaking of unity of humanity and the absolute equality of men and women. Bahais see themselves working towards the establishment of a world government which will eradicate extremes of wealth and poverty. The writings of Bahaullah are treated as sacred. It is estimated that there are between 3 to 4 million Bahais in the world today, spread in most countries of the world with the largest concentration in India. In Iran the Bahais remain the largest minority group with about 300,000 adherents. The international Baha'i center is in Israel.
Their leaders have made extraordinary claims to divinity similar to other religious cults. Although they seem to preach peace and unity, their history has been marred by violence. Their history and original teachings also contradict their averred concept of world peace and gender equality.
4. Shias 
Also spelled "Shi'ites." The "Twelver Shias" believe that, after the death of the Prophet, the Imamate (the political and religious leadership of the Muslim community) should have gone to 'Ali - the cousin and son-in-law of the Prophet - and his descendants as a divine right.
Unlike the Sunnis, who perform prayers five times a day, the Shi'ites pray three times a day. The Twelver Shias population in 1980 was estimated to be 73,000,000. They are dominant in Iran, but are also found in Pakistan, India, Iraq, Lebanon, Kuwait, Saudi Arabia, Bahrain, and Syria. There are also small Shia communities in the West, one of the largest in Dearborn, Michigan.
A religion based upon a claimed love of the family of Prophet Muhammad has lead them to beliefs contradicting the very essence of the message brought by him, the message of Islam
5. Nation of Islam 
The Nation of Islam was founded by Wallace Muhammad in Detroit in 1930. The group believes that a person called Fard Muhammad was "God on earth." It sees Elijah Muhammad as the "Messenger of Truth." Warith Deen Mohammed, the son of Elijah Muhammad, brought the group closer to mainstream Sunni Islam. Some dissatisfied members were led by Louis Farrakhan, who revived the group in 1978 with the same teachings of Elijah. They only allow people of black ethnicity in and believe they are the original race on earth. They are especially popular in the prison system in the US.
Founded by Dr. Rashad Khalifa, an Egyptian computer scientist. Submitters consider Rashad Khalifa to be a Messenger of God. They reject two verses of the Quran, preach the "miracle of 19," and reject the hadith and Sunnah of Prophet Muhammad. They are based in Tucson, Arizona, US, and are prominent on the Internet. They are considered completely out of the folds of Islam, due to their erroneous beliefs.
7. Sufis 
The most controversial and confusing "sect" would be the Sufis. In the West alone, there are more than 1000 Sufi sects. They are a very diverse group. Some Sunni Muslims adopt certain Sufi ideas, while other sufi orders have close links to ancient mystical orders. Yet, others have developed their own teachings and adapted them to a Western audience. Still others just use the term "sufi" but declare they have no relation to Islam or any religion whatsoever.
Generally speaking, they misunderstand Islamic spirituality and make errors in many key Islamic concepts like proper trust in God, love for the Prophet, and exaggerate the position of pious deceased Muslims. In terms of rituals, some will hold "Islamic chanting circles" ("zikr" circles), religious dancing like the whirling dervishes of Turkey, and keenly celebrate the birthday of Prophet Muhammad.
 For more information, you can view this link: http://www.islamreligion.com/articles/1736/
 For more information, you can view this link: http://www.islamreligion.com/articles/309/
 For more information, you can view this link: http://www.islamreligion.com/articles/490/
 For more information, you can view this link: http://www.islamreligion.com/articles/656/
 For more information, you can view this link: http://www.islamreligion.com/articles/1388/