Desktop or Towe Case
A desktop or tower case is required to hold all your components together. It is your personal preference on which one you decide to choose. Desktop or tower cases come in standard ATX form factor. You can find them in many different design & sizes. The most common sizes are Mini, Midi and Full tower. Most budget cases come with PSU (Power Supply Unit) but most of the premium cases don't come with PSU which has to be purchased separately. The case below is an ATX Midi Tower case.
To decide if you need a mini, midi, or a full tower case you need to ask yourself how many hard drives, dvd-rom etc you need to fit in the case. The bigger the case, the more bays you will have for your drives and peripherals. If you are building a pc which has to be small as possible and you are planning to buy a Micro ATX Motherboard then go for a Mini Tower case.
Note, if the case is really small and cramped then it will have less air flow, which means the case temperature would be higher than having a larger case with good air flow. This could lead to an unstable PC.
Recommended ATX Cases:
NZXT Guardian Black SECC Steel Chassis ATX Mid Tower Case 921RB-BL
Cooler Master HAF 922 - Mid Tower Computer Case with USB 3.0 Ports (RC-922M-KKN3-GP)
Corsair Obsidian Series Black 650D Mid Tower Computer Case (CC650DW-1)