Bitch is a commonly used slang word that is nonetheless vulgar when you throw it at a woman or a man. The word originally meant, and still means, a female dog.
This word for a female pooch has evolved to a more common meaning: a woman you don't like. In those cases, a bitch is kind of a female jerk, although feminists would argue that the term often gets unfairly thrown at any strong willed, assertive woman. Many males call each other bitch, too, implying that the other guy is a weakling. Another common use is bitching or to bitch and moan, meaning to complain about something.