When a muscle in your body tenses up painfully, it's a cramp. If you get a cramp in your leg when you're swimming, you have to stop and stretch out the muscle until the cramp goes away.
Muscle cramps are a side effect of exercise and fatigue, and they can often be avoided by stretching before and after a run or a game of basketball. Cramp is also a verb, meaning both "contract painfully," and also "hold back, or prevent free movement." This is what you mean when you say, "Mom, please don't walk me in to school — you'll cramp my style."