Yarster: Look on my website (address in sig). I’ve got a long essay discussing these phony critics and their laudatory quotes.
Regarding the original question: In my experience, these star ratings represent sort of a social average. Obviously any given critic will rate a movie differently based on his or her individual taste, and the general population will likewise have widely varying responses.
Recent example: The Thin Red Line. I thought it was brilliant, as did most critics; but a lot of people I know didn’t care for it, and general audiences responded with mystification. Four-star movie? Yes, for me; no for most people.
The people who put together television guides and movie-channel schedules take this into account. They’ll assign a star rating based not on the opinion of one critic (or a handful averaged together), but on the basis of the general population’s taste. An acknowledged classic like West Side Story or The Maltese Falcon gets four stars, sure, but a high-quality recent movie that divided audiences, such as The Thin Red Line or Eyes Wide Shut, will get a compromise three-star rating, and a difficult “niche” movie will occasionally get knocked down to two (Robert Altman’s The Player).
If you want a short definition, go with this: For any movie released after, say, 1970, the number of stars in your average TV or movie guide represents a cross between (1) critical response and (2) the proportion of ordinary viewers who say they really liked it – weighted toward the latter.
Examples: The Godfather is obviously a four-star movie; it’s an acknowledged modern classic, and what’s more, the majority of regular people who watch it really enjoy it. Contrast with American Ninja – it’s a one-star movie, because most people will hate it, but there’s a niche audience that gets off on that sort of thing.
Personally, I think stars are stupid. But that’s another essay – which, coincidentally, is also on my site. 
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