Chicken gyros and chicken souvlaki are very similar in flavor and both can be served in pitas with tzatziki so they are easy to confuse.  The main difference between the two is the cooking method: Traditional gyros are cooked as a turning vertical cone of meat and then the meat is thinly sliced off.  Souvlaki, on the other hand, is cooked horizontally on a spit over flames, so basically, our modern-day kabob.  Some souvlaki are served on the skewer, but they can also be taken off the skewer and made into a souvlaki pitas with tzatziki.