Before revision:  In Ethereum, once compiled, the smart contracts run on the machines of miners who can earn Ethers (a cryptographic currency like Bitcoin) by contributing their computing resources and thegas (in Ether) corresponds to the execution fee compensating such computing resources.
After revision:  In Ethereum, once compiled, the smart contracts run on the machines of miners who can earn Ethers (a cryptographic currency like Bitcoin) by contributing their computing resources and thegas and the gas corresponds to the execution fee compensating such computing resources.