The word tapas comes from the Spanish verb tapar (which means to cover). A tapa was meant as a free snack to be placed on top of a drink to keep flies and other what-nots out. While many bars provide a tapa with a glass of beer or wine, it's by no means the norm. More typically, going out for tapas implies ordering a plate of food called a ración, if it is to be shared among a few people.
Just about any manner of Spanish food comes in the form of tapas, and as such it's a very good way to go about trying the huge variety of Spanish dishes. Don't worry if you don't understand the menu, most tapas bars have their goods on display at the bar so you can simply point at what looks appealing to you.