Wall; A Pokemon that is meant to take either physical or special attacks over and over. (Do you know the difference between physical and special moves?) They often carry 1 or 2 offensive moves, a recovery move and a status move. The prime special wall is Blissey. She takes special attacks over and over and uses a recovery move so she can keep taking those hits.
Sweepers; These pokes are meant to just do damage to the opponents team, but that's about it. Their life goal is either kill or be killed. They almost never run a recovery move, and tend to have at least 3 attacking moves. The fourth move may be another attacking move but may be a move that boosts an offensive stat, such as swords dance, dragon dance or many others.
Tank; This is kinda a mix of a sweeper and wall. They are meant to take hits and hit back. They tend to not be as sturdy as walls and deal less than sweepers, but they can be a nice mix for any team.
Clerics are pretty much walls that carry moves like heal bell and wish. Heal bell will remove status from your entire team while wish heals whatever Pokemon is on the field next turn by half the original pokes health.
Any questions yet? I'm willing to answer them all