In addition to the good answer by "اللهم صل علی محمد و آل محمد" I'd like to consider it from another angle.
There are a few scholars from whose teachings today's main groups emerged. All of them lived in a relatively short era of time and more than 200 years after Prophet Muhammad (s.). In this era many Islamic schools of law emerged but the ones remaining today are based on the teachings of this five individuals.
- Ja'far al-Sadiq (702 – 765)
- Abu Hanifa (699 – 767)
- Malik ibn Anas (711 – 795)
- Al-Shafi‘i (767 – 820)
- Ahmad ibn Hanbal (780 – 855)
All of their teachings are based on Quran and Hadith. They differ in the question: What was the Prophets view on X? None of them wanted to found a new direction of Islam. They were teaching Islam as they understood it.
The first one is considered the sixth Imam of the Shia Muslims. Shia jurisprudence became known as Ja'fari jurisprudence (He is also a major figure for Hanafi and Maliki schools of law).
The other four schools of law (Hanafi, Maliki, Shafi'i, Hanbali) are considered as Sunni schools. This is what the first answer mentioned as
The interesting part is that Abu Hanifa and Malik ibn Anas learned from Ja'far al-Sadiq. Malik was also a student of Abu Hanifa. Shafi'i was taught by Malik ibn Anas for many years and Ahmad Ibn Hanbal learned from Shafi'i.
There are Muslims who don't consider themselves as following ONLY one of these schools of law, but take all or many of the different opinions of these five into consideration. Other groups who may still consider themselves Muslims are not based on any of these five schools of law. The question is whether they are still considered Muslims by the majority or not.