In a 4-3 defense, which the Bucs run, there are three different linebacker positions – the MIKE, the WILL and the SAM.
The MIKE is the middle linebacker, who is generally more of a run-stopper than someone used in coverage. The MIKE is also responsible for calling out the plays to the defense and helps the defensive linemen get properly aligned. For the Buccaneers, Bruce Carter (No. 50) and Kwon Alexander (No. 58) have both seen time as the starting MIKE linebacker.
The WIL linebacker is the weak side outside linebacker. Lavonte David (No. 54) fills that role for the Bucs. The WIL is usually the fastest of the three linebackers and can cover a lot of ground. Coverage is also a primary responsibility of the WIL.
For the SAM, the strong side outside linebacker, their responsibilities are a little bit in between. Generally, they're somewhere in between the MIKE and WIL in terms of size and speed and they play the run and pass equally. The Bucs' SAM is Danny Lansanah (No. 51).
Of course, specific responsibilities vary depending on the play call and situation and not every team's linebackers fit this mold, but those are the prototypes generally associated with those positions.
