A quad-core CPU has four processing cores in a single chip. It is similar to a dual-core CPU, but has four separate processors (rather than two), which can process instructions at the same time.
Quad-core CPUs have become more popular in recent years as the clock speeds of processors have plateaued. By including multiple cores in a single CPU, chip manufacturers can generate higher performance without boosting the clock speed. However, the performance gain can only be realized if the computer's software supports multiprocessing. This allows the software to split the processing load between multiple processors (or "cores") instead of only using one processor at a time. Fortunately, most modern operating systems and many programs provide support for multiprocessing.
Some examples of quad-core CPUs include the Intel Core 2 Quad, Intel Nehalem, and AMD Phenom X4 processors. The Intel processors are used in Mac, Windows, and Linux systems, while the AMD processors are only used in Windows and Linux systems. While four cores may seems impressive, some high end computers have two quad-core CPUs, giving them a total of eight processing cores. Now that is some core power!