A tomboy is a girl that doesn't really act like a girl. Tomboys don't really have to wear guy clothes, but they can wear things that are more relaxed to them opposed to girlie girls who will wear skin tight clothes and high heels a lot. Tomboys will not act like most girlie girls. They will make their own decisions and they really don't want or need anyone elses opinions. They can be involved in more rough activities, like football or hockey, and can be more interested in mechanics, computers, video games... tomboys don't really care about dirt; they can wear the same pair of jeans for days and not really give a dang. They do tend to hang around boys a lot, but they might also have some other tomboy friends. Sometimes if they have a girlie girl friend, it's could be because it's a long time relationship, or maybe it's a friend of one of their friends. Tomboys are not to be confused with stock show girls. Some tomboys tend to catch the attention of boys because they happen to have a lot of common intrests with boys. Some tomboys may notice, but don't really care.
Some reasons to cause a girl to be a tomboy would be that she grew up with a lot of guys around, like if she has a few brothers or if she grew up just with her dad.
I'm a tomboy. I grew up with just my dad and always got on good terms with guys instead of girls. I have four best friends. Two are tomboys, like me, one of them is a guy, and the other is a stock show girl.