The best place to start for info on styles is the BJCP website. Those are the official guidlines for beer styles and are full of information about why and how a style has a certain characteristic.
edit: from the bjcp guide for Imperial IPA
History: A recent American innovation reflecting the trend of American craft brewers “pushing the envelope” to satisfy the need of hop aficionados for increasingly intense products. The adjective “Imperial” is arbitrary and simply implies a stronger version of an IPA; “double,” “extra,” “extreme,” or any other variety of adjectives would be equally valid.