1 ether equals 10^18 (1,000,000,000,000,000,000) wei. Wei is the smallest indivisible unit; ether is the canonical user-facing unit.
1 ether equals 10^9 (1,000,000,000) gwei. Gwei is the unit gas prices are typically quoted in.
JavaScript numbers cannot represent integers larger than 2^53 without precision loss. ETH balances exceed that — 1 ether is already 10^18 wei. Native BigInt arithmetic gives exact, EVM-faithful values.
They are intermediate denominations: 10^3, 10^6, 10^12, and 10^15 wei respectively. Sometimes called babbage, lovelace, microether, and milliether. Rarely used in practice but defined in the protocol.