A computer is made up of several main parts. Obviously, the most important is the CPU, or central processing unit. This is the part that houses the computer memory and processing chips, in other words, the computer's brain. Most CPUs now have a CD and/or DVD burner, that is, a device for recording onto a CD or DVD, already built into the unit. They also have USB ports, that is to say, sockets where you plug in other devices, e.g. your scanner or memory stick. Most CPUs have software, i.e. computer programs, already loaded when you buy the computer, but many people like to add other programs to personalize their computer.