Clears are used as a defensive stroke to allow you time to regroup; and to move your opponent as far away as possible. A clear can be hit overhand or underhand, but both shots are hit as hard as possible and high.
Drop shots are soft shots that barely clear the net and then drop quickly into the front of your opponents area. They are used when your opponent is in the backcourt to force them to come up close to the net.
The smash is a very powerful shot that is used for getting the shuttle to land on your opponent's side of the court as quickly as possible, like a spike in volleyball.
A drive is a shot that is used to force your opponent to the back of their playing area. It is usually used when they are close to the net and is a hard hit straight shot that does not arc much.