This entry is just going to be a quick run-down of why tapas are the greatest things ever.
Tapas are often eaten in place of dinner, and can best be described as a roving cocktail party. You can start around 7pm, Spanish "happy hour," which is nice for us hungry Americans who haven't gotten used to eating dinner at 9:30 pm yet. If you wander into any restaurant in the city, you can pick (via pointing) a small dish and order a glass of wine. This costs around 2.50 euros, so $3-4. If you get around 3 of these, you will have had a full dinner's worth of food and 3 glasses of wine, all for less than $12.
Some common tapas are patatas bravas, which are fried potatoes with a spicy mayo-based sauce. Also common is tortilla, a giant omelet containing potatoes and other veggies. Bocadillos de jamón (small ham sandwiches) and fried eggs and chorizo over more potatoes.
Tapas are a fantastic way to hang out with friends, since you just walk around the city in twilight for an hour or two while enjoying good food, drink, and company. Basically, it's an ideal way to spend an evening in Spain.