Desktop: A Desktop computer is a personal computer (PC) in a form intended for regular use at a single location, as opposed to a mobile laptop or portable computer.
Laptop: A laptop, also called a notebook, is a personal computer for mobile use. A laptop integrates most of the typical components of a desktop computer including a monitor, a keyboard, a touchpad and speakers into a single unit
Netbook: A Netbook is a small, lightweight computer, smaller than a Laptop Computer, with a long battery life and ideal for travelling. Net book computers have built in Wi-Fi and are optimised for browsing the Web and Email.
Tablet: A tablet personal computer or tablet PC is a tablet-sized computer that also has the key features of a full-size personal computer. A tablet PC is essentially a small laptop computer, equipped with a rotatable touch screen as an additional input device, and running a standard (or lightly adapted) PC operating system like Windows or Linux.
PDA: A personal digital assistant (PDA), also known as a palmtop computer, or personal data assistant is a mobile device that functions as a personal information manager. Current PDAs often have the ability to connect to the Internet.