From LXF Wiki
CPU is an abbreviation for Central Processing Unit, one of the main functional blocks of any computer. The CPU is the 'brain' of the computer, that executes instructions, and reads and writes data to memory.
These days, the CPU has become like a mini computer in its own right. As well as the circuits required for processing instructions, they often include their own local memory, used for caching data and instructions.
Most modern PCs have CPU's based on the Intel x86 instruction set, made either by Intel or by a rival such as AMD. But there are many other CPU designs around, and Linux runs on most of them.
Probably the most important of these is the ARM family, used extensively in embedded and mobile applications due to its low power consumption and modular architecture allowing all the circuitry for a complete computer-powered device to be built on a single battery-powered chip. In fact, there are more ARM devices in the world than x86. The ARM family are unusual in that ARM Ltd (The Acorn spin-off that designs it) does not manufacture them, but licenses manufacture around the world. One of the biggest licensees is Intel, who have added patented memory cacheing technology to create the StrongARM (or whatever it's called nowadays) family.