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 the (in Ether) corresponds to the execution fee compensating such computing resources.