There are many variations, but most commonly it's made of extra-virgin olive oil and various herbs and spices.
The most common ingredients (other than the olive oil) are basil, minced garlic, bay leaf, oregano, rosemary, salt, and pepper. Many variations are possible, however. Some people add balsamic vinegar, crushed red pepper (fresh or dried), pignoli, and even finely-chopped prosciutto or hard salami.
My opinion is that simple is better. I just use olive oil, basil, oregano, and a tiny bit of salt. Once in a while, I'll mix in some grated Romano cheese, if I'm in the mood. But the oil shouldn't overpower the bread. Think balance.
-Rich