What are Wei, Gwei, and Ether?

Denominations of Ethereum's currency: Wei is the smallest indivisible unit, 1 Gwei = 10^9 wei, and 1 Ether = 10^18 wei.

Solidity has no decimals, so all value arithmetic is done in whole wei — "1 Ether" in code is 1e18, and msg.value is always in wei. Gwei is the common unit for quoting gas prices.