Shias are another sect of muslims. Within the sect of shias there are more sects which differ amongst each other. For example Iran is a shi'ite country.

However many Sunni muslims consider some shia sects to be out of the fold of islam hence ....kafirs (non-believers). But not all shias are the same so some of them are still within the fold of islam. A recognisable trait of a shia is in the islamic month of muharram they mourn the deaths of Ali (as) and on the news you often see pictures of them beating themselves till they bleed as a sign of their grief. However such practices are stronly forbidden in the sunni tradition. I should add that only 10-15% of muslims worldwide are shia (if I'm wrong please correct me). But in the main they are a minority sect, the majority being sunni.

In a nutshell The shias follow Ali (as) he was the prophets (pbuh) cousin, and son in law. and the father of Muhammad's (saw) only descendants. He was the male head of Muhammad (saw)'s household, after Muhammad died. Shi'a Muslims believe that Ali should have followed Muhammad as the direct successor and leader of the Muslims. Sunni's believe that Abu Bakr (ra) the first caliph to hold power after Muhammad, held his office legitimately. Basically its this difference that has created many minor and major differences in the customs and practices of these two sects today. In fact some shias have their own version of the qu'ran (so I've heard, but not too sure).