For basics: Gas is the amount of Ether one pays as the internal cost of executing a transaction or contract in Ethereum. In detail: Gas is a measurement that is roughly equivalent to the computational effort required to execute a transaction. Every transaction is required to include a gas limit and a fee that a user is willing to pay per gas; miners have the choice of including the transaction and collecting the fee or not. If the total number of gas units used by the computation spawned by the transaction, including the original message and any sub-processes, exceeds the gas limit, then all changes are reverted, except that spent gas cannot be reclaimed. For more info on Ether check our Resources section.