Why Does my jaw crack whenever I open it? Please help!?

Okay, So whenever I like open my jaw wide it like cracks and makes a click. Sometimes it gets so painful and hard to like open it fully wide then it makes a big crack noise ( click noise). What could this be? I have braces by the way if that helps. And if this helps I had a overbite when i was younger then i got it corrected by having a cross bow. to align it to the right position. Does this have anything to do with this? My friend says its lock jaw? but i dont know what lock jaw is, i know it has something to do with teatnus but im not for sure. Thanks !