One is designed to float, and the other to sink. In the case of wet flies, the hook is made of metal, and so the weight causes it to sink. In some cases, lead wire is also added, so that it sinks faster.

Most dry flies will not float by their own means, or will not float for a long time if you don't add a couple of false casts to dry them. Dry flies are made of a lighter metal, which helps their flotation.

Most of these flies are stiff, with a lot of hackle around the hook. Hackles also allow the weight of the fly to be dispersed across a wider span of water surface. It also causes resistance in the air, helping the presentation of the fly in the water. Finally, it contains a floating liquid, made of silicon paste or silicon with a solvent. This helps the fly float for a longer period.