The power supply, 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 gold standard in PSUs, these power supplies have fully modular cables which means you need to connect the cables your PC needs, resulting in a less cluttered, cooler-running and better looking PC.
Hybrid modular PSUs tend to have only the most essential cables permanently attached to the PSU, while extra PCIe, SATA and Molex connectors are supplied as modular cables. They're usually cheaper than fully modular PSUs, and are a cost-effective way of achieving a clean and tidy build.
These traditional PSUs (power supply units) have no detachable cables, making them less flexible than modular models when it comes to cable management, but they offer the best value for money. Look out for extra features too like high efficiency ratings, sleeved or flat cables, LED lighting and quiet fans.