The Graphic Interchange Format was widely used when the web was in its infant years because GIF files are relatively small. Each GIF file has its own palette defined, which must consist of fewer than 256 colours. GIFs can also include transparent areas, and also animation - although the animation is quite basic, with each frame being stored separately.
The big drawback with GIF that took it out of the market was the fact that the GIF technology was owned by Unisys, who insisted on demanding royalty fees. Sites such as www.burnallgifs.org sprung up, encourage web developers to spurn GIFs to make the point that software patents are bad.
The patent on GIF expired in the US in 2003, and will expire elsewhere in 2004 - while some will return to them, most will not because the community created a free replacement for GIF, known as PNG. GIF is pronounced "Jif".