Zone defense is different from man-to-man defense in that, instead of guarding a particular player, each zone defender is responsible for guarding an area of the floor, or "zone", and any offensive player that comes into that area. Zone defenders move their position on the floor in relationship to where the ball moves.
Zone defense is often effective in stopping dribble penetration and one-on-one moves. On a personal note, I believe that all kids must develop their man-to-man defensive skills first. I believe youth basketball leagues should limit the use of zones to the older age groups.
On the other hand, some high school and college coaches treat zone defense almost as if using it were blasphemous, or an admission of inferiority! At the upper levels, I believe you should assess your team's strengths and weaknesses as well as your opponent's, and the game situation, and use whatever tool you need to try to win.
Advantages of zone defense
1. Not all teams have quick, good man-to-man defenders. Or the offense may have a couple of outstanding players too quick to defend individually. Playing a zone can help against mis-matches such as these. You may have a tall, strong, but not particularly quick team. A zone can look intimidating with all your big guys stacked up inside with their hands up.
If you have a very short guard or two, in playing man-to-man defense, the opponent will run screens to get switches and size mis-matches against your guards in the paint... you can help avoid this with a zone defense by keeping your small players out on the perimeter.
2. No lay-ups. In using a zone, you can protect the paint area and force the opponent to shoot from outside. An example would be using the 2-1-2 or 2-3 zone which places a lot of defense in the paint and invites the offense to shoot from outside. This is a good way to see if the other team can shoot from outside. Not all teams can shoot the outside shot consistently. Even good shooting teams have off nights, especially under tournament pressure.
3. Your team may be in foul trouble, especially your big man. You can help protect him with a zone.
4. You can slow the game and control the tempo somewhat with a zone.
5. Opponents tend to become impatient against the zone and often rush shots.
6. Most youth and high school players are just average passers (at best), and have difficulty making good inside passes and accurate skip passes.
7. By changing defenses from man-to-man to various zones, you can keep the offense off-balance and confused.
8. If you use the full-court or half-court press, after the press is broken, it is easy to slip back into a half-court zone defense.
9. Compared to man-to-man offenses, there are far fewer zone offenses to contend with, and zone defense can easily be adapted to counter the opponent's star player(s).
10. If you play zone defense exclusively (no man-to-man), you can save practice time in not having to teach how to handle ball-screens and other screens.
Disadvantages of zone defense
1. If your team is behind, you won't get enough pressure on the ball, and the offense can eat up a lot of time by holding the ball for a good shot. You must go man-to-man in this situation.
2. If the opponent is having a good shooting night, your zone is beaten, and you must consider going to the man-to-man to get pressure on the ball out on the perimeter.
3. There are not always clear-cut rebounding box-out assignments and sometimes the offensive player will slip inside for the offensive rebound and lay-up. See Zone Rebounding for help with this.
4. If you play zone most of the time, and rarely play man-to-man, your players may become complacent on defense and may lose their man-to-man skills. I would practice man-to-man 80-90% of the time, and zone the remaining 10-20%.
Basic pointers for all zone defenses
1. No lay-ups. Try to keep the ball outside. Double-team the ball in the paint.
2. Be vocal, talk to each other.
3. Move quickly, adjust your position relative to the movement of the ball. Move quickly on the flight of the ball, as soon as it leaves the passer's hands.
4. Get your hands up and out, to shrink the passing lanes.
5. Close-out on the shooter with high hands, to pressure the shot and the pass. We are less concerned with dribble-penetration since the zone creates too much congestion inside for the dribble-drive.
6. Stay in your defensive stance. This is especially important for weakside defenders. The weakside low defender should keep his "butt to the baseline" so that he can see the floor, and see any cutters or screeners coming his way, etc.
7. No fouls... play good defense without fouling.
8. No second shots... be aggressive rebounders.
9. When the offense dribble penetrates, quickly close the gap.
10. Get to know your opponent and adjust. Over-protect against the best shooters, or the "hot" shooter, and sag off the guy who never shoots.
11. Trap the corners.12. Especially if you are ahead, don't gamble or get too zealous about trapping the wing and point guard positions. Keep pressure on the ball, but also protect the paint and force the outside, low-percentage shot.