This is what I found on a quick search.
Sometimes called charley horses -- particularly when they are in the calf muscles -- cramps are caused by muscle spasms, involuntary contractions of one or more muscles. In addition to the foot and calf muscles, other muscles prone to spasms include the front and back of the thigh, the hands, arms, abdomen, and muscles along the rib cage.
Almost everyone experiences muscle cramps, which come without warning.
Spasm vs. Cramp
The difference between a muscle spasm and a muscle cramp is the force of the contraction. If a muscle quickly contracts and releases without pain, it is a spasm. A prolonged and painful contraction is a cramp.