This is a good question and there will be many differing opinions on this topic. With that said, there are a few points that most people will agree upon. First, Orientalism is the study of things from the Orient, that is to say, the East. It is in distinction from the West, which is called the Occident.
Second, most scholars agree that the Western study of the East not only divulges information about the East, but it is also a window of what the West is like. It shows the preoccupations of the West and their patterns and categories of though. Edward Said is the probably the best known author on this topic. His book, which is a must read, is called, Orientalism.
Finally, within literature, the nineteenth century is seen as a hotbed of studies of the East, but it is of continued importance and interest today.