Gold – This is generally "mixto" or adulterated tequila, where colors, sugars and flavors have been added to the fermented, distilled agave juice before bottling. It's unaged, sometimes called "joven" or "oro" and tends to be on the less expensive side, so it's what's often used at high-volume bars and restaurants. There's a good chance you can credit at least one of your youthful tequila hangovers to this free-flowing stuff, so while it's not necessarily bad, you can certainly do better.
Silver – This clear tequila is un-aged and may also be a mixto, so look for the words "100% Agave" on the label. It plays nicely with other ingredients, and tends to make for a bold, but smooth margarita. Silver tequila may also be labeled "blanco."
Reposado – A nap sounds good, right? This one has been "rested" in a wood barrel or cask for at least two months but less than a full year in order to age, and possibly pick up characteristics from a barrel's previous occupant - cognac, wine or bourbon. It may be mixto or 100% agave, but it's almost guaranteed to be a bit more complex than a younger tequila.
Añejo – Tack on a bit more time. An añejo must be stored for at least one year in a vessel that doesn't exceed 600 liters, and generally mellows to a deeper amber color, with an even richer flavor. Extra añejo or "ultra-aged" tequila hangs out in the container for three years minimum and can often hold its own against other aged spirits like bourbon, scotch and cognac.