Girls don't exactly just walk around and give hugs to guys. But if she does hugs you or kiss you on the cheek ect, she may be flirting. Unless you two are dating, then she is just showing you affection, so don't act like it's strange.
A hug is something girls use to be affectionate towards guys without losing their stealth in the process. Boy and girl friends do it too- but it usually depends what your relationship with the girl is.