VIX itself is an index. Most of the instruments that track VIX such as UVXY, VXX, TVIX all track this index through buying front month VIX futures. These futures track the volatility in the market and rise in value when IV goes up, and fall in value when IV falls. These instruments also have something called contagno, in that they naturally decay over time as the are continually buying and selling VIX futures. This is the main reason why if you don't know what you are doing, don't touch it.