Pita bread is flat, circular stuff that has more-or-less separate top and bottom crust, so you can open it up and put your fillings in the "pocket" you've created. What you put inside us up to you. I prefer the same kinds of things I put into deli sandwiches, but you could easily put in stir-fried veggies, spaghetti, or whatever else strikes your fancy.
Opening up the pita bread is a little tricky the first few times. Slice the bread in half across the diameter. Then, carefully insert a non-sharp knife, like a butter knife, into in the center, where the top and bottom crusts are separate, or at least where the bread is thinnest. Work it around to open it completely, but don't slice through the edges (or the sides, for that matter) or your sandwich will tend to leak or fall apart. Some people heat up the pita a little before slicing, as that can soften the bread, but don't go too far and toast it.
I've also used mini-pitas to make little two-bite
sandwiches. You have to use a fairly homogenous filling
(like tuna salad) in them, of course - cramming
lettuce leaves into a two-inch-diameter pita isn't
going to work so well.