Crystal glazes are glazes where crystals grow in the liquid glaze as it solidifies in a cooling kiln. If you've seen a matte glaze on a pot, then you've seen a type of crystal glaze. There are "two" types of crystal glazes: those that form crystals so small that the glaze becomes matte, and the macrocrystal (or visible crystal) glazes.
The pot above is glazed using a macrocrystal glaze. The effect is of crystals that formed over the entire surface of the pot with a shape like a bundle of sticks. Below are three pictures of three glaze samples glazed with a different formulation of the same glaze on the blue pot. Due to the different clays the glaze is applied on and the different chemicals in the glaze, the crystals formed to create very different appearances.