Carpet is usually nailed to the floor I believe. Rugs as sergil said is smaller and can be moved from place to place.

A rug is smaller then carpet. It used for covering floors. Especially in front of doors or a fire-place.

To elaborate on what everyone else has said, A carpet is something that covers the entire floor and is generally nailed or stapled to the floor. It never moves unless the homeowner decides to get it ripped up. A rug can be small or large, but it is mobile.

A small carpet and can be moved from one place to another but the rug is large