The old style of power supply is now called non-Modular. Notice the mess of cable that are permanently attached to the power supply. Well if you don’t need them all, which is likely, then you have to wrap that mess up and tie it somewhere inside the case.
In contrast, here is a modern Modular power supply:
Notice that there are no cables hanging out of this PSU at all. You only use the cables you need to run your system. Modular power supplies are a little more expensive, and they are only provided for larger sizes. 550 Watts is about the smallest Modular PSU that’s available.