1. Lying, spreading rumors, or gossiping about someone behind their back.
2. Responding in a promiscuous manner to "cat calls" or "sleeping around". (Hollering back, or Hollering from her back.)
3. Having wealthy or politically connected parents. ("Hollaback" connotes a wealthy suburb of the city referencing the "Hollywood" neighborhoods of pop culture soap operas like "Beverly Hills 90210", "Melrose Place", "Party of Five", "Dawson's Creek" or "The O.C.".)
Said girl is widely presumed to not have the character to face their accused in good faith or a fair fight.
Due to the popularity of the Gwen Stafani song in 2005 and the general acceptance of suburban white teens it is no longer an exclusively "urban" term and if used will surely identify the user as someone who is hopelessly behind the curve. The only legitimate use of the term as of the summer of 2005 is as about oneself in a self effacing manner to elicit humor or disdain for contemporary pop culture buzzwords or phrases.