Gas

Gas refers to the fee required to add a transaction for execution on the Ethereum blockchain. Gas prices are quoted in fractions of Ethereum’s native token ETH referred to as GWEI.

A billion GWEI = 1 ETH

If a GWEI = a penny than ETH would = $10,000,000  

GWEI is a blend of the words giga and wei and occasionally you’ll see gas prices quoted in WEI instead of GWEI.

A billion WEI = 1 GWEI

The gas price is determined by user demand, spiking when network volumes are high.

When executing transactions users define a gas limit. The gas limit reflects the maximum amount they are willing to spend to have their transaction included in a block. The higher the gas limit the more quickly a transaction will be included in a block because of the greater reward to miners for confirming it.

Further Reading Gas

Gwei Definition and Explanation