Central Processor Socket
Obviously you can't have a computer system without something running the show, and the computer's central processor (often referred to as the CPU or a microprocessor) is traditionally considered to be the brains of the entire operation. In the labeled-parts image, the CPU socket is the grayish-white unit within a black outer border; on modern motherboards, this socket is commonly set up for easy-installation. Of course, when installing the CPU it pays to align the corners properly as the socket can tell if you're trying to put it in backwards; so if you're up against socket resistance then you're obviously doing something wrong. Usually the notched or dotted corner (as shown in the image to the left) is the key to a successful installation, so take note of this marking and plan accordingly.
The aforementioned black border area functions as a mount for the CPU heat sink, the installation of which is a necessary part of any computer system as it helps to cool down the system; leaving this part certainly has some rather dire consequences. Installation of a fan unit on top of the CPU further degrades the heating of the computer system, and should also not be overlooked; additionally the CPU fan needs to be plugged in to the marked power port on the motherboard in order to function, as shown in provided image.
Other types of processors and similar chipsets exist as well; however, these are usually built into the motherboard and are generally not user-replaceable. These include the southbridge unit, integrated graphics processor and Ethernet units, and the onboard sound processor.
(Section Image: Example of an Intel Core2 Duo microprocessor; image credit: Wikimedia Commons/Simial.)