[HUbison]

As you know, in a normal 4-3 defense, there are three linebackers covered by four defensive linemen up front. The weak side linebacker is the player on the side opposite the tight end. The weak side backer is often referred to as the "Will" and is usually your fastest player at the position. He has to be able to cover the pass well, often against a running back and sometimes even a receiver. A good way to look at the weak side linebacker is to think of him as the roamer. He has the freedom to chase the ball as much or more than any defender on the field. The middle linebacker usually has the most responsibility. More often than not, he is looked at as the guy to call the plays and signals to the defense. He has to be versatile, meaning he needs to be able to drop into coverage and play the run. It's preferable to have a big, physical presence in the middle, but the more athletic he is, the better. That's why a guy like Brian Urlacher is so successful. The strongside linebacker essentially has to be the most physical presence among the linebackers. He doesn't do as much pass coverage as the other spots and is generally the player asked to blitz against the pass. Speed off the edge and the ability to hit are most important for the strong side backer. He usually has to take on the tight end.

This is a pretty good explanation on the different roles of the different LB positions. It's quite the misperception to think there are two types of LBs....MLB and OLB. As Waggoner points out, just saying OLB is not a good description of the position. Think about it like this. In general terms (not always, but usually), the stat leaders at LB should break out like this:

Will (WOLB) - passes defended (Spoon led the league in this last season, by the way, and Pisa was 9th)
Mike (MLB) - tackles (8 of the top 9 tacklers among LBs last season were "Mikes")
Sam (SOLB) - sacks (8 of the top 10 sack leaders for LBs were "Sams")