Also known as the CPU (central processing unit) by Intel and CPU and APU (accelerated processor unit) by AMD, this is the computer's "brain," responsible for processing information. Performance is the most important factor, and is determined primarily by the number of cores it has and its clock speed.
Intel and AMD are the dominant processor manufacturers. Within each company's product lines are various processor families. Intel's include the Atom, Celeron, Pentium, and second- and third-generation Core; while AMD's include the Sempron, Athlon II, Phenom II, E-Series, A-Series, and FX. Intel's new CPUs are the third-generation Core i3, i5, and i7. They're setting a new standard in desktop performance by adding an automatic speed boost when needed and improved graphics and video capabilities. These processors are also available in laptops. AMD's new processors are called Fusion. The A Series of Fusion integrates discrete graphics into the processor. As a result, you can play more challenging video games on these computers without needing separate graphics.
Processors with multiple cores can process more data simultaneously. You can often tell how many cores a processor has by its name. A Core 2 Duo has two cores and a Core 2 Quad has four. A Phenom X3 has three cores. It's not always that clear, however; some Core i5 processors have two cores and others have four. The Core i7-980x has six.
Clock speed, measured in gigahertz (GHz), determines how quickly a processor can process information. Generally within a processor family, the higher the clock speed, the faster the processor. Clock speeds typically start at around 1GHz for a mobile processor and can exceed 3GHz for a desktop processor.
Power consumption is another important factor when choosing a processor, especially for laptops--lower power consumption translates to longer battery life.
When buying a computer, make sure it has a processor that will be fast enough to handle your needs. Whether you're buying a desktop or a laptop, avoid computers that use the AMD Athlon Neo, Turion Neo, or Sempron processor, and the Intel Atom or Celeron processor. For basic tasks such as browsing the Web and checking e-mail, opt for a low-end dual-core processor such as the Intel Pentium or AMD Athlon/Turion X2. If you plan to use your desktop or laptop for entertainment, such as watching videos or playing games, get a faster processor such as the Intel Core i5 or AMD Phenom II or A6. If you're a gamer or plan to edit high-definition (HD) video, buy a computer with a high-end processor such as the Intel Core i7 or AMD A Series. For less-intensive uses such as productivity tasks, the Intel Core i3 should suffice. For a netbook, stick to the slow but efficient Intel Atom processor.