Gas (Ethereum) is the internal pricing (metering system) for running a contract or in general any transaction in Ethereum. Gas is a unit of measurement for the amount of computational effort required to perform operations on Ethereum. The more computation and storage needed to complete a transaction, the more gas that is required to perform and complete that transaction. Metering the the amount of Gas required per operation. For example, addition (3 Gas), multiplication (5 Gas), storage operations (200-20,000 Gas), or stack operations (2-3 Gas) all require different amounts of Gas.
Ethereum uses Solidity to run its smart contracts on the Ethereum Virtual Machine (EVM) and every line of code that gets executed in Solidity requires a specific amount of Gas. Gas is paid to miners on the Ethereum network who perform necessary computational work to execute transactions. The purpose of implementing the Gas system to complete transactions using Ethereum is to incentivize miners to perform computation. Gas and Ethereum work together to complete a prof-of-work peer-to-peer system where miners mine blocks and get block rewards through becoming temporary dictators of their own mined blocks on the blockchain.
The fee paid to miners is the gas used multiplied by the Gas price. The Gas price per Gas by default is 0.02uETH (micro ether) per Gas, but Gas price may also be specified by the user. If a higher fee (price per Gas) is set by the user, then this increases the reward for miners which will likely increase the speed at which transactions with higher fees to be processed. This Gas price system allows users to pay more for faster transaction completion times on Ethereum.
Optimizing your Solidity contract's gas usage - Coinmonks - Medium
What is Ethereum Gas: Step-By-Step Guide - Blockgeeks
Documentaries, videos and podcasts
All About Ethereum - Gas
April, 7th, 2017
Ethereum, Gas, Fuel & Fees
June 2, 2016