The power-supply unit, or PSU, powers all the components inside your PC. The main thing you need to know when shopping is the approximate wattage your system will consume, so roughly tally up the power consumption of your various components. It is recommended to over-spec a little to ensure reliable operation, and get a little extra for future upgrades. Also check that the PSU comes with enough connections for your build, and that it will fit inside your chassis. Modular PSUs allow you to minimise excess cabling by only having the connections you need attached, meaning your PC will not only look more professional but will also run cooler. The quality of a PSU can be indirectly derived by looking at its 80 PLUS rating, which defines its efficiency, with Bronze, Silver, Gold, Platinum and Titanium categories indicating ever-increasing power efficiency.
