Есть нормальное объяснение. Но на английском:
Gas is provided in the transaction that activates the contract as two variables "gas" (amount) and gasPrice (in wei) - currently the minimum gasPrice is 10 szabo. Gas is consumed by each instruction in the contract (some cost more than others) until it is depleted or the contract terminates. When the contract terminates, any unused gas is returned to the sender.
This gasPrice mechanism allows the ETH/gas ratio, or the cost of execution, to be set to a level the network will accept on a per transaction basis. In theory this automatically drives the transaction fee down to the actual (hash-power-weighted average) cost of processing a transaction.