Mike - The 'M' represents the 'M' in middle linebacker. In a 4-3 formation, the Mike (sometimes referred to as the 'Inside' or 'Mack' linebacker) is the middle linebacker. In a 3-4 formation, the Mike is weak side middle linebacker, since there are two middle linebackers. (Weak refers to the non-tight end side, strong refers to the tight end side.)
Will - The 'W' represents the 'W' in weak side inside linebacker. In a 4-3 as well as 3-4 defense, this is the inside linebacker on the weak side.
Sam - The 'S' represents strong side linebacker. In a 4-4 defense the strong safety plays just like a linebacker, but also has the option of dropping back into the safety position if the offense lines up with a one, or no back set. In a 4-3 scheme, the Sam linebacker plays the strong side. In the 3-4, the Sam is the outside linebacker on the strong side.
Ted - The 'T' represents the 'T' in tight end. In a 4-3, the Ted is not used. In a 3-4 and 4-4 scheme, the Ted is the middle linebacker on the strong side.http://en.wikipedia.org/wiki/Linebacker