In brief, Said's groundbreaking book Orientalism made two main arguments: "the Orient" is a false construct invented by the Europeans, particularly England and France, and secondly, this construct was invented to control a large number of ethnicities that were considered "other" and exploitable by the European powers.
Said contends that what Europeans call "the Orient," as if it were one vast monolith or single culture, in fact includes a great variety of markedly different cultures, stretching as it does from Turkey and Egypt across to India and then to China and including all the countries in between. What, for example, Said askes, does Turkey, a Muslim country in Europe, have to do with China, a Confucian nation half a world away in China? What do they really have in common? Likewise, what does Egypt have to do with Japan? "The Orient" is simply a myth, he argues, a false construct created by Europeans for their own convenience.
Said also asks why the Europeans would do this. He argues that reducing all these cultures to one faceless mass made it easier to stereotype them as one vast inferior group and, subsequently, control them. In one fell swoop, the inhabitants of countries as far-flung as Syria and Korea were depicted as mysterious, childlike, treacherous, feminine, and, most importantly, in need of European mastery and control. Orientalism then became a convenient tool helping to justify the colonization, exploitation, and imposition of "superior" European ways on these nations.