The PSU (a.k.a. power supply unit) is virtually the heart of the computer. It is a vitally important part that delivers power to components that comprise the computer system. A high-quality power supply can make the difference between a perfect machine and an unreliable one. Getting to know the importance of the power supply is just the first step, and getting the right PSU for your system means looking beyond just enormous power output.
You need to consider the physical size of the power supply, and whether or not it fits your case's form factor. How many "rails" does it have, and how many peripherals it can connect with. Look at placement of your cables and whether they'll fit into your case, and if you can run them to all the places your PSU must connect. Properly chosen, your power supply will last you a long time!