I think the following website gives a good, brief summary of the main difference between Shiites and Sunnis:
Differences in Shiite and the Sunni Sects
To summarize:
The various Shiite groups believe that there is a person appointed by Allah to be leader of all Muslims, and who must be obeyed in all matters (kind of like the Pope). But there is not agreement about who the current leader is, which means there are many different branches of Shiites.
The Sunnis don't believe in the (current) existance of any divine leaders. So, roughly speaking, using Christianity as an analogy, we have:
Shiite = Catholics
Sunni = Protestants
To give a few examples of Shiite groups and their current leaders:
Ismailies - follow the Aga Khan
Ahmadiyyas - follow Khalifatul Masih IV
"Twelvers" - this is by far the largest group of Shiites, and includes most of the Shiites who live in Iran and Iraq. They do have senior clerics, but there's no agreement about "who is in charge".
A case in Iran from a few years ago really illustrates the emphasis Shiites place upon their leaders. A university professor named Hashem Aghajari was sentenced to death for saying, in one of his classes, that Iranians aren't "monkeys who should blindly follow religious leaders". Because of this statement, he was found guilty of blasphemy and sentence to death. And just to be safe, the Iranian legal system also fined him, sentenced him to beatings, and also gave him a long prison sentence.
After a couple of years in jail, Hashem was finally released. You can find a good summary of his case here:
Professor Hashem Aghajari Finally Freed from Jail