The white wire is neutral, the black wire is “hot”, or live. A green wire is an earth ground wire, which provide a path back to the source of the electrical current (the main electrical panel) in case of a fault, which should trip the breaker. Both white and black wires carry the current.
It is good practice to make note of which wire is on which side when replacing. If replacing a receptacle, the white wire goes on the silver screw. The bare copper or green wire goes on the green grounding screw, and the black wire gets attached to the brass screw. If the terminals aren’t easy to identify by color, the ground terminal might be marked with a “GR.”, the white wire goes to the side with the longer slot, the black wire always goes to the short slotted side.
Before working on anything electrical turn off the circuit breaker that controls the outlet and test that the power is off with a circuit tester.