The kind that dress like a guy, have an attitude like a guy, but still have cute feminine features in their face that they can't hide. I knew a girl in high school like that, I liked picking on her a little. She'd say "I could kick your ass in arm-wrestling" and I'd say "but you're too cute to arm-wrestle :3" and she'd get beet red and pissed.
It's pretty hot to see them dress all boyish but once they take their clothes off, they have soft curves and perky tits.