A witch is a girl and a wizard is a boy. The power is the same. (depending on how much the witch/wizard studys or practices magic). In Harry Potter, usually everyone just calls themselves wizards, but occasionally they use the word "witch" for girls. A witch and a wizard would marry.
If you ever saw the tv series bewitched from the 1960s the female was a whitch and the male was called a warlock. This can be confusing when you try to distinguish it from the Harry potter movies when add on the fact that both male and female wizard and whitches ride on brooms also on the tv series the warlock was more powerful.but the male is a wizard and the female is a witch. I could go on and on but the traditional continuity would be more confusing!