Souvlakis in a pita are the hamburgers of Greece. At least they were until hamburgers arrived and now hamburgers are the hamburgers of Greece. Souvlaki shops have become more scarce and fast food places more plentiful, though you should not have trouble finding a souvlaki if you want one.
What is a pita?
It is usually meat of some sort, beef, lamb, pork or 'unknown' that is on a giant vertical rotisserie. The souvlaki 'cook' slices off the meat as it becomes done and puts it on a round pita bread with lettuce, tomato, onions, and sadziki, a cucumber-yogurt-garlic sauce that in my opinion is what makes souvlakis great. In some souvlaki shops the meat is skewered and sometimes you can get ground beef, which is generally known as beefteki. In some upscale souvlaki shops you can get chicken souvlakis and even swordfish souvlakis(Very rare. I think there is a place next to MacDonalds at Syntagma where I saw it). It comes in wax paper which keeps it all together and you eat it like you are peeling a banana.