The international tourist industry has caused some misunderstanding, if not outright misrepresentation, in the noble concept of the tapa.
Unfortunately, in the touristy areas of large Spanish cities, some local bar owners have adapted the original concept to fit in with the expectations of visitors, so you will now see bars announcing themselves as Tapas Bars. That, by definition, cannot be the case.
Allow me to elaborate. A tapa literally means a lid or a cover. In the south of Spain it is customary when buying an alcoholic drink in a bar, to be given a small plate of something to eat. This may consist of a few olives, a couple of small fried fish, some cheese, cold meat, potatoes in 'alioli' sauce, etc. That is the tapa. The idea behind it is that it is free, and given at the discretion of the waiter. One is not expected to ask for it, or express a preference for one thing or the other. In some bars, the waiter considers it a part of his art to choose the tapa most appropriate to the drink being consumed, and the taste of the consumer. I have heard a theory from a reliable source that the name tapa came about because the little plate was placed over the glass, like a lid, possibly to prevent flies from getting into the drink. It's an attractive notion, but unfortunately not corroborated by any of the major Spanish reference dictionaries consulted.
What many visitors actually mean, when they refer to tapas, are pinchos. Pinchos are more common in the north of Spain, in Bilbao and San Sebastian for example. Pinchos are small portions of 2 or 3 bites, normally impressively laid out in lines of trays on the top of the bar. In this case, the client would order a drink, and ask for specific pinchos, which would then be charged for.
Another thing altogether are restaurants where it is normal to order a number of different dishes, which are then placed in the middle of the table, with everyone dipping in freely. These are raciones, not tapas. A fine example of such a place in Madrid is La Trucha, famous for being the preferred hang-out of top bullfighters. If you go there, don't miss the exquisite fritura de pescado (assortment of fried fish) or the champinones rellenos (stuffed mushrooms).