While almost every device is a full-fledged system
The central processing unit, or CPU, is the key brain of the entire system that enables it to work. This is much like the brain of a human that retains memory and passively and actively does calculations for every task in everyday life. Computers today are evolving by adding more cores, which are basically entirely separate processors in one single chip. There are three different stages of cache memory that are used comparable to system RAM, but with faster access to the processor and system itself. The idea is that these stages of cache will store frequently requested processes and use that information rather than reprocessing it again and again. The level 3 cache will usually be more plentiful but be slower, whereas the level 1 cache will be speedier with less storage space.
The system RAM is the next stage of memory, but serves a slightly different function than cache. When a program loads, it is actually being transferred from the hard drive to the system's RAM for speedier reading. Without RAM, computer systems would be significantly slower than they are now considering that all of the reading for the programs would have to be done by the hard drive itself which happens to be very slow. When the computer system is turned off all the information in the RAM is erased, and because of that a hard drive is required to permanently store information.
A hard drive's only job is to store permanent information on the computer system for storing. While the other components of hardware are not necessary to make the system work, they typically provide a needed operation. The network card allows a personal computer to connect with a network of systems to share files or get to the Internet. While data centers require many Cisco GLC-T or other SFP transceiver modules to function efficiently, most home systems get away with using one single Ethernet port per system for communication. Usually a sound card and video card need to be installed to create a picture to see and sound. Motherboards are built to connect everything together and process information between them with simpler integrated chipsets.