Twelvers
Twelvers refers to the group of Shias who believe in twelve
Imams. They constitute the majority of current Shias. They are also konwn as Jafaris, Imamis, Rafidis, and by other names. But in general, when talking about Shia, it is usually in reference to the Twelvers since they are the "orthodox" variant of Shiism.
The Imams are viewed to be infallible and are similar to prophets. The last imam is believed to be hidden and will return, he is called the Mahdi.
For more information see: Shia