If you take a look at the illustration on the right, you will see a diagram outlining the 4-4 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 near the line of scrimmage (imaginary line seperating the offense and defense). You have two defensive ends (DE), one on each end of the line, and two defensive tackles (DT) in between. Spread out behind the defensive line are the four linebackers (LB).
Two cornerbacks (CB), one on each side of the field, line up to cover the wide receivers. There is just one safety. The exact position of the defensive backs (cornerbacks and safety) depends on the type of pass coverage they are in.