A long straddle buys a call and a put; both options have the same strike price and expiration date. A long straddle would be executed if the trader expected the underlying stock to make a big move but wasn't sure of the direction.
A short straddle sells both a call and a put with the same strike price and expiration. The maximum profit for a short straddle is the premium received. The maximum potential loss is infinite if the stock rallies.