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 or "zone" and any offensive player that comes into that area.
Zone defenders move their position on the field in relation to where the ball moves. Zone Defense is often effective in stopping penetration and 1v1 moves because as a ball carrier dodges he makes his way into more zones and encounters more defenders. Zone tends to make slides more simple for defenders to learn and is very effective for promoting team defense. Zone Defense also helps defenders recognize and block passing lanes since they are more concerned about positioning and relation to teammates than with chasing an opponent around the field.
Tactically within a game a Zone Defense can be used to slow down the tempo of an opponent's offense. A team with a lead, for example, might switch to zone because an offense will not have as much success with individual run and gun dodges against a zone and will instead need to operate with passing and teamwork, which likely means it will take longer for the offense to develop shooting chances.