The power supply unit provides electricity to the internal components of your computer and to devices connected to it, such as input devices, USB hubs, card readers or any other external device that does not require a dedicated power cable. While the basic architecture of a PSU does not change drastically from computer to computer, you have many options for things like cabling and positioning.
The Basic PSU
First of all, we have the basic PSU that you will find "bundled" with a computer case. This PSU has several power cables coming out that terminate with what are called "Molex" connectors, which are asymmetrical to ensure that they are oriented correctly when inserted into a device. These cables have specific terminators to plug into the motherboard, disk drives and video cards. Furthermore, there are ATX and BTX varieties. BTX PSU's are installed in the bottom of a BTX case, while ATX PSUs are installed at the top of a case.
From here, you have the option to purchase a PSU with detachable cables. This is called a "modular" PSU. Often, you will not need all the cables coming out of the PSU, and it's handy to remove the extras, rather than leaving them dangling or having to figure out a way to tuck them into a corner. You don't want a cable to brush up against one of the fans in the case, but you also don't want it to obstruct other cables you may need access to later.
PSU cables can also come "sleeved." This is a flexible netting that consolidates the cables, which is a good compromise when the PSU is not modular. The netting is also favored for cosmetic purposes, since PSU cables are brightly multicolored. For this reason, most sleeves are black, but you can order them in a variety of colors, and you can order a "sleeving kit" if none have been put on your cables.
Some PSUs oriented toward performance enthusiasts use a different shape, which emphasizes temperature management. These PSUs also emphasize power efficiency (electrical flow is reduced as temperature increases), support for multiple video cards and low noise. These performance enhancements are largely implemented by increasing the length of the PSU, which gives its internal fan more room for better air flow. Therefore, some types will not fit in smaller computer cases.
Laptop and Netbook PSUs
Laptops and netbooks use an external PSU that does not contain a fan. This is a large block that's encased in thick plastic or rubber. These are generally much cheaper than internal PSUs and supply much less wattage (since a laptop doesn't usually require as much power as a desktop PC).