By itself,
homeboy can simply mean a
black person, especially one who dresses, talks, or acts
ghetto. (But you need not be born
black to be a
homeboy.)
When you call someone your homeboy, you're saying a lot more. Your homeboys are your clique, your crew, the people who are down with you. Your homeboy is someone you love.
Homey (homie) is a related term, but sometimes implies a more casual relationship.