With lightning speed, a computer carries out millions of calculation each second. Sets of instructions called programs tell the computer what to do. The hard-disk unit is the heart of a computer. It contains the central processing unit (CPU), which controls all operations of the computer. The hard disk unit, monitor, keyboard, and other connected devices are called hardware. The programs that enable it to function and carry out specific tasks are known as software. Every computer has a program called an operating system (OS) which controls its basic functions. The OS is always at work “behind the scenes” when other applications are running. Most schools and many homes have personal computers, but there are many other types of computers, both larger and smaller than a PC (personal computer). Some computers enable people to work while they are traveling, and others are designed purely for entertainment. Large, powerful, high speed computers are often used to process information for many people at once, or perform many tasks simultaneously. Many business people carry small portable PC's called the laptops with them when they travel. A laptop contains rechargeable batteries that allow it to function on trains, buses, outdoors, in fact almost anywhere.