True tequila is made from agave. If a tequila isn't 100% blue agave, then it's supplemented with alcohols and flavors from other sources (usually corn). If you taste a 100% agave tequila, you'll see why tequila was invented: it's fruity and pungent, not harsh.
Like most good liquors (brandy, port, scotch), it benefits from time to develop its flavors. If you leave it for two months to a year, it's "rested" (reposado). Leave it for longer than that, and it's "old" or "aged" (añejo). As with scotch, the longer you let it rest, the more expensive it is.
You can let it rest in glass or steel tanks, which will leave it clear (silver or "blanco", white). Or you can rest it the way they do with whiskey, in charred wood barrels. That gives it other flavors (caramel, vanilla, tannin, pepper, etc. depending on the kind of barrel), and turns it a golden color ("oro").
For cheap tequilas, not made from 100% agave, they make "gold" tequila by adding caramel colorings and flavors, but it bears little resemblance to the properly aged gold tequilas.
If you try the good tequila, you'll find that the cheap stuff is intolerable. A good tequila is every bit as complex and interesting as a good scotch. Unfortunately, it's also about as expensiv