ATX is the standard for PC power supplies. It has +3.3 V, +5 V, and +12 V. They share a common ground. It has a PS_ON# signal which needs to be grounded to turn it on. Don't open power supplies that run on line voltage (AC). They have dangerous voltages and currents inside

It stands for "Advanced Technology eXtended". There are low-current -12 V and +5 V (called 5 Vsb) that are always on, even when the main supplies are off.