A female who doesn't conform to societies idea of what "feminine" is because she just happens to not like it. Tomboys are girls who are pretty on the inside and the outside, it just so happens that they don't feel the need to show it off. They also get along with males incredibly well, and never finds it "awkward" to be attracted to the opposite sex in a friend-type way. tomboys are also great friends. Generally, tomboys are the girls who like having fun by playing video games/sports/etc. and seriously don't give a crap if their hair isn't the exact way it was when they left their freaking house.
When a tomboy is friends with a girly-girl, she will sometimes be begged into wearing a skirt. After agreeing (because tomboys are excellent friends) she will end up complaining about it, because, turns out, skirts are worse than imagined.