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Cases...
This is a computer case. Computer cases come in many shapes and forms, some small, some big. Some look basic, some look artistic. At the end of the day, when building a PC, choose the case that suits you. You can't really go wrong when choosing one. Just make sure that the case you buy is the right size to fit your motherboard into. For example, if you have a ATX motherboard then you must get a ATX case (most cases are ATX), if you get a Micro-ATX motherboard then you must get a Micro-ATX case (pretty much all small cases). Also when choosing a case make sure it has enough hard drive bays for the amount of hard drives you want to use. Also make sure it has enough bays to fit the amount of optical drives you want. 
Sources: General Knowledge, WikiPedia. The image of the case was taken by one of our team members.