Ether is the internal network currency for Ethereum. Bitcoin has Satoshi and Millibits, and the US dollar has dollars and cents, Ethereum has its own names and terminology. Since Ether is used to pay for various transactions and computational tasks performed on the Ethereum network, these actions are called GAS. Each block has a GAS restriction (that is, how many computational transactions can be performed per block), which limits the block size and immediately determines the cost of executing the contract.
All small bills are named after the pioneers in the field of cryptography, as well as in the terminology of the Bitcoin name. Finney is named after Hal Finney, Szabo in honor of Nick Szabo, Satoshi in honor of Satoshi Nakomoto.