Italian seasoning is just a mix of different dry herbs commonly sprinkled for an Italian taste. It is used in a long list of dishes-- from the famous spaghetti and pizza to soups and many others. This one from McCormick contains marjoram, thyme, rosemary, savory, sage, oregano and basil.
In supermarkets here in Japan, I've seen it as a simple "herb mix" which contains less herbs.
I've also made my own Italian seasoning with my five favorite herbs: basil, oregano, thyme, sage and rosemary.