The strength of the 3-4 is its ability to confuse the quarterback during passing plays. Most teams generate a pass rush by sending at least 4 defensive men at the quarterback. In a standard 4-3 alignment, these 4 rushers are usually the 4 down linemen. But in a 3-4, the fourth rusher is a linebacker. Since there are 4 linebackers, the fourth potential rusher can come from a variety of spots on the field, thus influencing and sometimes confusing the quarterback's pre-snap defensive read.
The 4-3 is widely used because of its balance in stopping both runs and passes as well as being personnel-friendly since smaller players can be used effectively along the defensive line while the 3-4 is much more personnel specific.
A drawback of the 3-4 is that without a fourth lineman to take on the offensive blockers and close the running lane, both the defensive linemen and the linebackers can be overwhelmed by blocking schemes in the running game; thus a 3-4 defense requires a large and strong nose tackle able to routinely tie up 2 or more blockers, freeing the middle linebackers to make the tackle. The 3-4 linebackers must be very athletic and strong enough to shed blocks by fullbacks, tight ends, and offensive linemen to get to the running back.
In traditional Cover 2 schemes the free safety and strong safety have deep responsibilities, each guarding half of the field leaving the linebackers and cornerbacks in man to man or zone underneath coverage. Teams that play Cover 2 shells usually ascribe to the "bend-but-don't-break" philosophy, preferring to keep offensive players in front of them for short gains while limiting long passes. The main weakness of the Cover 2 shell occurs in the middle of the field between the safeties. The safeties attempt to gain width upon the snap of the ball to cover any long passes to quick wide receivers down the sideline. This movement creates a natural hole between the safeties that can be attacked. By sending a receiver (usually a tight end) into the hole, the offense forces the safety to make a decision: play the vulnerable hole in the middle of the field or help out on the wide receiver. The quarterback reads the safety's decision and decides on the best matchup (which mismatch is better--TE vs S or WR vs CB).
Tampa 2 blends the Cover 2 and Cover 3 defenses by having two defensives backs, usually the safeties, in deep coverage on either side of the field, and a middle linebacker covering the medium to deep middle. Its benefit over the Cover 2 is that the sidelines and middle of the field are better protected against deep threats, with the drawback being a larger open area in the short middle of the field underneath the middle linebacker. Its benefit over the Cover 3 is that it only dedicates two defensive backs to deep coverage rather than three, allowing for better protection against short outside routes.
