Graphics Interchange Format, Version 89a
Commmonly known as GIF, this is a bitmapped image format widely used on the Web. Options include "progressive display" in which the rendering exploits interlaced lines, permitting recognizable images to appear before the whole file has downloaded; and short animations that exploit multiple images and control data within a single file. GIF uses LZW compression and palette-based color (256 or fewer shades).
According to Wikipedia (as of August 29, 2008), "CompuServe introduced the GIF format in 1987 to provide a color image format for their file downloading areas, replacing their earlier run-length encoding (RLE) format, which was black and white only. GIF became popular because it used LZW data compression, which was more efficient than the run-length encoding that formats such as PCX and MacPaint used, and fairly large images could therefore be downloaded in a reasonably short time, even with very slow modems. ... The optional interlacing feature, which stores image scan lines out of order in such a fashion that even a partially downloaded image was somewhat recognizable, also helped GIF's popularity, as a user could abort the download if it was not what was required."
The ironically titled "Sad day . . . GIF patent dead at 20" includes a useful chronology compiled by an individual unhappy with Unisys's patent protections as applied to LZW compression.
The Wikipedia article also includes information about PNG (a format specifically designed to succeed GIF and to avoid patent problems) and MNG (a variant of PNG that supports animation). PNG was never widely adopted, probably because it arrived coincident with the availability of browser support for JFIF (the file format for JPEG_ENC encoding), and because the LZW US patent was scheduled to expire in 2003. MNG was never directly supported in browsers.