Photo courtesy Rick Cooper of Flickr under Creative Commons license
Vugs are the name given to small cavities in a rock. The typical vug is like this example from Gold Hill, Nevada: an opening in the middle of a vein, lined with crystals of a mineral (in this case, quartz) that is different from the material around it. A cavity in lava that represents a former gas bubble is usually called a vesicle, but vesicles that become lined with crystals are typically called vugs. Vugs tell a general story of an opening being formed by some mechanism, which is then partially filled with mineral material, perhaps by another mechanism and perhaps at another time.
Vugs are prized by mineral collectors and gem miners, who may also call vugs "pockets." The word comes from Cornish, where it referred to an underground chamber or cavity.