Hawkmoon is based on a series (well, actually two series) of Novels written by Michael Moorcock, one of the pioneers of the heroic fantasy genre.
The RPG takes it name from the main character of the Hawkmoon books, Dorian Hawlmoon von Koln, and it set in the distant future. In than future, Europe has reverted to a mostly feudal, pre-industrial state, although some bits of technology remain, in some cases far in advance of what we have today.
The central story in the Hawkmoon series is that of Hawkmoon and his allies attempts to stop the evil Empire of Granbretan (Great Britain, but one where the people are cruel, twisted, perverted, sport beat masks, and are committed to conquering Europe).
That a very brief lowdown.