PDAs

It is a small, hand-held computer that can be used as:

• as a PC
• for web browsing
• as a movile phone
• as a portable media player       

Some PDAs have a touch screen and are controlled using a stylus

laptops

Many people work on the move these days and use portable computers. some laptops are small and light, while others, have large screens that are useful for work or watching films.

NETBOOKS have built in wifi access, webcams, speakers, and microphones. they are small and light. they save wight by not having a cd drive. However, netbooks can be fiddly to use because of their small keys and screens.

The latest portable computers have enough battery life to allow someone to work most of the day on one charge.

Personal Computers (PCs)

Most people are familiar with general. These computers can run a wide variety of programs, whic make them to different things, for example, word processing or creating electricity bills.
the PC ( personal computer ) is a general purpose computer. It comes in various forms, e.g. a desktop PC, a laptop, a PDA or even a movile phone.

You can have more information on :  http://en.wikipedia.org/wiki/Personal_computer

what is a computer ?

a computer is an electronic machine that processes data. It follows instructions given to it in the form of a program.
Computers are everywhere these days and there are many different types.