"Pill" is a more general term. It can be either a capsule or a tablet.

If you're talking about medicine, a pill is a round and oval capsule, and a tablet is flat and circular.

Christopher's answer is the only correct answer. In British English, we say 'tablet' more often.

Pills are small solid "round" masses of medicine or vitamins that you swallow "without chewing". A tablet is a small solid mass of medicine which you swallow.

Pill is a single round capsule while tablet is a a collection of pills in a pack