A Computer casing is an enclosure or a shell that houses the internal components of a Personal Computer (PC or RIG). Aside from Computer case, there are lots of other names that may be used to refer to it as well, such as 'Case', 'Chassis', 'Enclosure' or 'housing' depending on the preference of the individual that is describing it. This is sometimes wrongly indicated as the CPU, especially by the general public.

There are many types of PC chassis available to users which may be categorised in a few different ways which is normally by the size of the largest Motherboard that it is able to house.

Recently though, this has gotten quite mixed up as there are more different sizes of motherboards than there were a few years back thanks to technological advancements. Having said that, the motherboard isn't the only component that affected the size of the PC chassis, more often than not you'll see chassis that are deeper or longer than normal even though it could only fit a normal ATX-sized motherboard; this is due to several factors such as Longer Power Supply Units, Bigger GPU, Taller CPU, User feedbacks or even manufacturer's naming schemes (such as Ultra-Tower named by Cooler Master to the CosmosII).

Arguably, there are so many things to consider when referring the size of a PC chassis. So in this thread, to save a lot of hassle, here is a general guide on the PC Chassis sizes:

Full-Tower - The biggest beast of them all! Normally with more than 8 expansion slots and is able to house motherboards larger than a normal full-sized-ATX motherboards up to large consumer-grade motherboards such as the EVGA SR2 without (or with minor) modifications

Mid-Tower - The most used size by consumers, has about 7 to 8 expansion slots that houses Full-sized-ATX motherboards or even the slightly larger BTX or E-ATX depending on the design.

Mini-Tower / Small Form Factor (SFF) - Compact chassis that houses Micro-ATX or smaller motherboards that has 4 or less expansion slots

Qualities:
Silent Operation
Controllable Fan Speeds
Digital Workstation Worthy (Audio and Visual)

Parts:
Motherboard [looking at the Asus x79 Deluxe right now. any comments?]
CPU [Intel 4770K?]
Graphics Card (Something that can be used in gaming and Video Editing)
Sound Card (Something that is of high quality for audio editing)
Case [Fractal R4?]
PSU (something that can supply enough power after overclocking)
Fans (Silent fans like Noctua fans depending on case)
RAM (I need enough for intense editing and also high in Hz)
Storage (preferably SSDs in Raid 10)
Optical Drive (should be a BluRay read/write with DVD read/write)
Cooling (Try not to be custom cooling because I'm not the kind of guy that will change the coolant)

Peripherals
Monitor (1080p Monitors are fine. If possible, another monitor in vertical orientation on the right side that matches up with the horizontal one.)
Keyboard (something functional and have great response)
Speakers (gonna need awesome monitor speakers)

Legend:
() Require reccommendation
[] Require advice on