This term is usually refered to a girl seems to embody
all that is popular at the moment. This could be used in context to all of mainstream society, or just one specific genre. They often become an "ideal" to aspire to. Sometimes they highlight something typically considered bland. Sandra Bullock
's popularity rose because she was considered "the girl next door
The "it" girl usually becomes a household name, at least for a week.. commonly, they disappear as fast as they appear. Occasionally they transcend a season, and are considered to "define" generation, era, or a sub-culture.
Good examples would be Molly Ringwald
, Kate Moss
, Britney Spears
, Bettie Page