A computer is a system of many parts working together. The physical parts, which you can see and touch, are collectively called hardware. Software refers to the instructions, or programs, that tell the hardware what to do.
Types of Computer systems
A Desktop computer is the type of computer people are most familiar with. It contains most of the computer components within its case, such as the CPU, the Hard disk and the RAM. A monitor would usually sit on the desktops case and an external keyboard and mouse would be attached to it.
A Laptop is a portable computer that is powered by mains electricity via an AC adapter; it can also be used away from an electricity supply using its rechargeable battery. A laptop has most of the same components of a desktop computer, a screen, a keyboard and a pointing device (Touchpad), these are built into the laptop and do not need to be externally attached.
Handheld computers or PDAs (Personal Digital Assistant) are computers small enough to store in a pocket and to use in the palm of your hand. Handheld computers are usually used for keeping schedule with calendars, using its address book and accessing the internet.