Most people will experience these awful cramps in the feet and legs (especially the calves), but since your muscles are all over your body (duh), a charley horse can technically happen anywhere. Fun!
As for why it's called a "charley horse," there's not a single accepted story, though it appears to be related to baseball. One legend claims it was named after an actual horse that limped around a baseball diamond dragging a brush behind it. Another story asserts it was named after Charley "Old Hoss" Radbourn, who suffered from cramps in an era when people had way cooler nicknames.
Whatever the origin is, when you get one, it sucks.