Case

See examples at Newegg. 

The case is the outside of a computer. It can be made from steel (strong), aluminum (light), or plastic (cheap).  Some cases are sold with a power supply already installed, while others require you to buy a power supply separately. Today almost all computer cases are made in China. Smaller cases can be cubes the size of a toaster or look like a stereo amplifier. Larger cases stand on the floor and may be mid-size or full-size "towers". Cases can be plain with simple lines, or gaming systems can have designs and transparent sides.

Things to look for in a case:

• Small Case - Looks like part of your stereo system if you hook it up to your TV and use it to record and play media. Small cases have small fans that may have to run at high speeds and make noise to keep the system cool.
• Big Case - Lots of wasted space for cards you don't install and disks you don't add, but that leaves air circulation. Large (120 mm) fans can move a lot of air at low speeds like 800 RPM where they make almost no noise at all. You can't hide this unit and need to make room for it on the floor.
• A removable motherboard tray allows you to assemble or change the mainboard, CPU, and memory on a table and then slide it into the case.
• More fans are better. The Antec Nine Hundred case has two 120 mm fans on the front, one on the back, and a giant 200 mm fan on the top. Run them all at low speed and you keep everything cool.
• Easy disk installation. In some cases, you attach a mount to the side of the disk and simply slide it into place. The problem with the Antec Nine Hundred is that you cannot add or remove disks without unscrewing and sliding out a subassembly attached to one of the fans. Of course, if you only have one disk, and never change it (and never back it up to another disk) then this doesn't matter.
• As all those fans move air, something in the system will filter out all the dust, lint, and dog hair. You want it to be a filter supplied by the case, because the alternative is that that gunk will clog the heat sink on the CPU and effect performance. The best systems have a removable, washable filter. 
• Get more bays than you think you need. A 5 ¼ bay can hold a DVD burner, Blu-Ray reader, or a dock into which you can plug and unplug bare SATA hard drives. An internal 3 1/2 inch bay holds hard drives. Yes you can buy an external enclosure that connects these devices to the computer using USB or External SATA, but it is simpler and more convenient to leave room for devices you decide to add later on. 
• The cheapest cases are stamped out quickly and have sharp edges. While you don't open the case and work inside a lot, it is unusual to build a new system in a $50 case without ending up with at least one Band-Aid. For more money you can get a better made case with no sharp edges, but how much is a cut finger worth to you in cash?
• A "tool-less" case has plastic brackets that allow you to insert disks or the DVD drive without screws, but learning to use a Phillips screwdriver is not really so hard.

If you want an entry level case that comes with a decent power supply, the Antec 44xx series is quite good. It typically sells for $70 but can be had for as little as $40 after a mail in rebate. The Antec Nine Hundred (without a power supply) can be $140. If you are looking for careful design rather than brute force, Lian-Li is the highest quality and most innovative vendor of computer cases.