There are usually two tackles. Some teams use this player on defense to help stop the run. He lines up right in front of the center. These players line up behind or in back of the defensive line. They are good at covering short passes, stopping the run, and blitzing.
Some teams use three and some teams use four linebackers.
A linebacker who lines up in the middle or even with the ball. A linebacker that plays on the outside of the defensive ends. These players like to blitz and can cover receivers well. A player who plays ten to fifteen yards off and even with the ball and helps cover the whole field. His main responsibility is not to let any player on offense get past him.