Gas is the fuel of the Ethereum blockchain network. Gas is always paid in ether. Gas is the transaction fee paid to miners for executing and adding your transaction into the Ethereum blockchain.

For every transaction, you have to specify the value of the gas price and gas limit you want to set. Based on these values, the network calculates how much of a fee you are willing to pay for a transaction.

Every bytecode operation in EVM has fixed gas units assigned. When that operation is executed by EVM, fixed quantity of gas is consumed by the transaction.