Yep just what timswi said. Basically it's a container that holds an amount of dessicant (sp). As air passes through it the dessicant pulls the water out of the air stream. The cheap ones are designed for paint sprayers and only last about a week. The good ones run about $45.00 and are more permanent. The top of the line are the refrigerant systems which basically is a small refrigerator and a coil of copper tubing with a drain. They work on the basis that when you cool the air the moisture will condense out of the air stream and is collected and drained. BTW; the dessicant systems "can" be rejuvinated by removing the pellets from the container and heating them in a microwave for about 1 - 2 minuets.