Just to add to echelon answer, gas represents the computational effort of a specific EVM (ethereum virtual machine) instruction. For example, deploying a contra

Just to add to echelon answer, gas represents the computational effort of a specific EVM (ethereum virtual machine) instruction. For example, deploying a contract cost a certain amount of gas, storing a variable another, etc... However when you send an ethereum transaction (i.e you modify data in the blockchain) you need to pay the miner in ether. You cant pay in gas. So you have to specify how much ether you are willing to pay for each unit of gas, i.e the gasprice. The combination of both (gas x gasprice) will give you the maximum amount of ether you are willing to spend for this transaction.