Lead crystal is a type of glass that's used to make high quality glassware for use with semi-formal and formal dining. Not surprisingly, it has lead in its composition, with 24% being the standard for decent crystal.
Lead crystal glass is often somewhat finer than regular glass, and noticably less coloured. Often the glassware has an intricate cut pattern in it, or perhaps some frosting.
The lead doesn't cause any health risk because of the way the matrix is built up, although some drinks shouldn't be left in it for a significant period of time. For example, it's safe to drink whisky out of a crystal glass (many won't drink it out of anything else), but over a long period of time, it can leach the lead, so whisky shouldn't be left in a crystal decanter.
NEVER put crystal glassware in a dishwasher. The extremely caustic detergent causes a reaction with the lead, turning the glassware cloudy.