In computers, the form factor is the size, configuration, or physical arrangement of a computing device. The term is commonly used in describing the size and/or arrangement of a device, acomputer case or chassis or one of its internal components such as a motherboard or a daughterboard. If you see the term applied to software or programming, it will usually mean the size of the program or the amount of memory required to run the program effectively. When used to refer to the size of a free-standing computer or other device, it's close in meaning to footprint.
Notebook computers, for example, are a form factor unto themselves because they all take the same "clamshell" form, are typically rectangular and open to a keyboard on the flat surface and a screen on the top. Notebooks are also available in a variety of form factors that identify a type of product and market area.
Here are a few different notebook form factors:
An ultrabook is a category of thin and light laptop computers designed to bridge the market gap between tablets and premium notebook PCs.
A netbook is a small, light, low-power notebook computer that has less processing power than a full-sized laptop but is still suitable for word processing, running a Web browser and connecting wirelessly to the Internet.
A convertible tablet is a computer that can function as either a standalone touch screen device or as a notebook with a physical keyboard.