Depending on context. In an AC mains power application the earth wire directly ties to a stake that's in the earth outside. This provides a safe return path for faulty circuit conditions.

Ground is a broad term in DC applications where it is the main negative' plane or hookup. However you can have ground in an AC application where it usually is the neutral return.

Generally green in mains power (Australia/NZ and I believe EU) is always the earth.