Buy drink, get free food.
I think a very smart person was sitting around one day when this idea came to him: Buy a beverage, get free food. BAM! Best idea ever. And it shall be called it "tapas".
Essentially, a tapa is a small portion of food that comes "free" whenever you purchase a drink at a bar/pub/tavern/restaurant/pretty much anywhere that serves tapas. This can range from seafood, sandwiches, meat plates, fries, veggie plates, etc.
Also, let me clarify that you get food with every drink you buy. After living in Florence, where drinks cost me 4-10 euro each, the offer of a drink AND food for 2 euro is equivalent to this kid getting an N64 or this kid getting a blastoise...
Tapas were EVERYWHERE in Granada. Now, there are multiple explanations for the existence of tapas - I'm sure its a combination of many different accounts. For practical reasons, it is believed that the original tapas were slices of bread or meat used to cover a glass of sherry to protect from fruit flies (hence 'tapa', the Spanish word for 'cover').
Sticking with practical explanations, the Spanish eat dinner relatively late - around 9-11pm - making it common to curb one's appetite by barhopping and socializing with friends after work. Another reason for tapas is to stimulate social interaction by taking the focus away from the act of drinking. However, another theory to the origins of tapas contradicts this one.
Tapas could be the brainchild of innovative bar owners. Often, the meat used to cover was ham or chorizo - both of which are salty and thirst-inducing...which means people buy more drinks...follow? K, good. Touche crafty bar owners...
There are more explanations, most of which less likely than the ones above. Regardless of how tapas came to be, one thing is for sure: TAPAS ARE AWESOME!