# Single phase Ac Wire voltages

I'm sorry for asking a question this simple but I can't understand it . Single phase ac has two wires , phase and neutral but I don't know which wire change voltage . Like neutral wire is always 0 v and phase wire changing +V to -V or phase and neutral changes direction ? (Like phase +V , neutral 0V then neutral +V , phase 0V ). Also what's the difference between pulse signal and ac ?