I see both of these terms used in restaurants that seem to specialize in meat on a vertical rotisserie served in a pita sandwich. I'm not clear on whether there is a difference between the two or if they are just colloquial names for the same thing.
They're rather different.
Gyros are Greek in origin. They are simply meat, tomatoes, onion, and tzatziki sauce on pita. In Greece the meat is typically pork (never had one). In America, specifically here in Chicago (their local origin), the meat is a combination of beef & lamb.
Shwawarma is Middle Eastern in origin. The possible toppings are much more diverse, and can include: tahini, tabouli, fattoush, cucumber, and hummus. Tzatziki isn't typically used, at least not in America. The meat is also never pork, it can be lamb, beef or chicken.
They owe their similarities to their common Turkish ancestor the doner kebab.
As sarge_smith hinted at in his comment to hobodave, at least from the gyros and shwarmas I've had.
- Gyros are made from a loaf of ground meat, onions and seasonings, spit roasted. (this might be a regional thing, though)
- Shawarmas (and doner kebab) are cuts of meat, marinated and stacked, then spit roasted.
As you slice the shawarma meat off the cone, you're cutting across the stacked slices, so it'll end up falling into lots of smaller bits, while the gyro meat is larger slices, but still tender because it's ground meat.
- Gyros tend to have more "mediteranian" herbs (oregano, marjoram, thyme, rosemary)
- Shawarmas tend to have more "middle eastern" spices (cardamom, allspice, cinnamon, cloves, coriander seed, tumeric)
- Either one might have cumin, pepper or oregano, and they'll both have garlic.
I don't know how much of it's a regional thing (as I think I've only had shawarmas twice in the US), but when we used to get shawarmas in the Netherlands, they were served with a garlic yoghurt sauce that was similar to tzatziki, but didn't have cucumber or herbs in it. They were also served inside pita pockets (with shredded carrot, lettuce and tomatoes) as opposed to wrapped in a flatbread.
I have many times been to Greece and have had gyros there and they are not ground meat but just like the shawarma it's pieces of meat stacked and roasted. Difference is just like the others say spices and the toppings that go into the pita. Also gyros in Greece are almost always pork meat.
Greek gyros is always cuts of meat, not homogenised with herbs etc. like a donner kebab, and comes as either pork, or less commonly, chicken. In the UK what is described as 'chicken donner' or 'chicken shawarma' is almost always very similar to what is called 'chicken gyros' in Greece in terms of the meat and its appearance, though spices etc are different and it will probably be served with different salads.
I'm a chef, trained by "off the boat" greeks, and gyro is lamb/beef..not pork, and I find it hard to believe that the Greek family that I was trained under would be making gyro wrong.. American style is typically in a "cone" meat form, and Greek style is stacked/layered meat, then sliced as the others said..souvlaki is typically pork, which is different than gyro..
Gyros are not usually pork. in Greece, they are wrapped lamb and beef with proper spices skewered and roasted. In America, I have made them with ground lamb and hambburger.