The CPU (Central Processing Unit) is a microchip, classed as the brain of the computer. Its job is to interpret the instructions that come from the hardware and software, so that the hardware and software can communicate with each other for example. CPUs have different types (Celeron, Pentium, AMD, i3, i5, i7 and so on) and different speeds (800 Mhz, 1.6 GHz, 2.5 GHz, 2.9 GHz and so on).

The CPU  -  A Microchip that processes software/hardware instructions

The CPU is one of the main reasons for buying a computer. The faster the CPU can process instructions the faster the computer. However, this is not strictly true. An Intel Pentium 4 is faster than an Intel Pentium 3, in theory. And the "stripped down version" of the Intel Pentium 4 (the Celeron CPU) is supposed to be faster than the Intel Pentium 3. I say supposed and in theory because it depends how you slow the computer down or speed it up.

A computer with an Intel Pentium 2 and 7200rpm hard drive installed is faster than a computer with an Intel Pentium 2 and 5400rpm hard drive installed. And OS X (Mountain Lion or Mavericks) with 8 GigaBytes of memory to use is faster than OS X (Mountain Lion or Mavericks) with only 4 GigaBytes of memory to use. So far this is as expected. However, you can have a computer with an Intel i3, 8GB of memory and a 7200rpm hard drive installed that will be faster than a computer with an Intel i5, 4GB of memory and a 5400rpm hard drive installed. This is because even though the Intel i3 processes instructions slower the instructions are being put into memory a lot quicker.

If a Microsoft Word file on the hard drive for example is being copied into memory, perhaps for printing and/or editing purposes, the copying might be done at 1MB intervals. Whereas when OS X (Mountain Lion or Mavericks) knows it has more memory it knows it can copy data at 2MB intervals for example. And with a faster hard drive too it means actions such as moving files, installing software and indexing files become a lot quicker. So the memory and hard drive make up for the slow-ish CPU.

With time and progress manufacturers have now made it possible for the CPU to be split up into two or four parts (cores), known as Dual Core and Quad Core respectively, which basically means the CPU now has two or four brains to use. These core CPUs spread tasks amongst themselves, making it possible for one brain (core) to print out a document for example whilst another brain (core) takes care of processing internet tasks for example. These kind of CPUs are becoming common in many computers these days due to technology getting cheaper and so on. Therefore, to some degree, CPU speed is not so much of an issue these days in terms of computer speed.