A dry iron presses your clothes with a hot plate against an ironing board. In contrast, a garment or clothes steamer typically doesn't require a board and is used on hanging fabrics. It straightens out clothing by shooting a continuous jet of steam while held at a distance from the fabric.
There are also steam irons, which incorporate the best of both worlds. The steam iron presses flat against your clothes while using little jets of steam to iron out those wrinkles.