Tower cases have either a single side panel which may be removed in order to access the internal components or a large cover that saddles the chassis. Traditionally, most computer cases required computer case screws to hold components and panels in place (i.e. motherboard, PSU, drives, and expansion cards). Recently there is a trend toward "screwless" cases, in which components are held together with snap-in plastic rails, thumbscrews, and other methods that do not require tools; this facilitates quick assembly and modification of computer hardware.
As shown in the image below -