The Power Supply Unit (PSU) converts AC power into the DC power that your motherboard and drives need. Its internal fan also provides essential cooling for the PC components and drives. ATX power supplies dominate the market and plug into all ATX (Advanced Technology Extended) and BTX (Balanced Technology Extended) form-factor motherboards. You will find up to 7 connectors on power supplies. Molex, mini, SATA, PCIe, P1, P4 (or ATX12V 2x4), and AUX - for drives, video cards, and motherboards, respectively.
Power supplies are rated in wattage, such as 400-watt or 1-kilo watt; plus better PSUs employ Active PFC (Active Power Factor Correction) to stop harmonics.
Power supplies die suddenly or slowly over time. With the latter, the clue is intermittent problems. Replace the power supply, don't try to fix a dead or dying power supply.