What is Compression Clothing?
Compression clothing can be any stretchy, form-fitting garment—from sports bras to knee socks to shorts or tights—made of a blend of spandex and other fabrics, such as cotton or polyester. The stretchiness of these pieces comes from the spandex or lycra content, as this material can stretch up to seven times its original size before snapping back into shape.

How it Works
No matter what form it comes in, compression clothing is designed to squeeze, hold, and control the muscles of the body, almost like a second skin. And, like skin, compression clothing serves a variety of functions, including:

*Decreased muscle oscillations, or in other words, greater support. Compression bras, shorts, and shirts help keep muscles—or muscles-in-training—under greater control and sleeker, making bodies more aero- or hydro-dynamic for faster times and increased comfort. The extra support, much like that provided by wrapping an ankle or knee, also stabilizes joints to prevent or alleviate injuries associated with sudden, uncontrolled movements.

*Moisture control. Most compression fabrics are woven into a breathable, easy-dry pattern that wicks sweat and other moisture away from the body quickly. This regulates muscle and body temperature, allowing wearers to withstand high temperatures to a greater extent. This moisture-wicking technology also prevents heat exhaustion and fabric mildew.

*Strategic surface pressure. Compression clothing also places pressure on specific points of the body, not unlike massage or acupressure. This stimulation increases blood flow, which helps speed muscle recovery for better, longer workout routines and improved fitness results.

*text taken from http://fitnessindustry.com.au/2010/05/19/compression-clothing-an-innovation-in-fitness/ (click for more information on compression clothing)