When a defender gets past the offensive line and tackles the quarterback, it is called a "sack."
The wide receivers flank the quarterback. Their job is to move down the field toward the end zone and make themselves open to receive a pass.
In a man-coverage defense, defensive tackles and ends cover the offensive line, linebackers defend the pass and the run, cornerbacks cover the wide receivers, the strong safety covers the tight end and the free safety stops anyone who might get past him. In a zone-coverage defense, each defensive team member is assigned a zone and stops anyone who enters it.