Gas is the execution fee for every operation made onethereum. Its price is expressed in ether and it’s decided by the miners, which can refuse to process transaction with less than a certain gas price.
Gas is the internal pricing for running a transaction or contract in Ethereum. At the time of writing before the launch of Frontier it is fixed to 10 Szabo, which is about 1/100,000 of an Ether. It’s to decouple the unit of Ether (ETH) and its market value from the unit to measure computational use (gas).
Thus, a miner can decide to increase or decrease the use of gas according to its needs, while if need be, the price of gas can be increased or decreased accordingly, avoiding a situation in which an increase in the price of ETH would cause the need to change all gas prices. This is also a response to the discussion in bitcoin about fees structure.