If you take a look at the illustration on the right, you will see a diagram outlining the 4-3 defense. The Os in the diagram represent offensive players while the Xs represent the placement of the defensive players.
Notice the lowest row of Xs on the line of scrimmage (imaginary line separating the offense and defense). You have two defensive ends (DE), one on each end of the line, and two defensive tackles (DT) in between. Right behind the defensive line are three linebackers (LB).
Two cornerbacks (CB), one on each side of the field, line up to cover the wide receivers. There are also two safeties. The exact position of the defensive backs (cornerbacks and safeties) depends on the type of pass coverage they are in.