GIF

GIFs are a type of computer image that have small file sizes and are quick to download. As opposed to the JPEG format, the GIF format can have a maximum of 256 colours so is poor at displaying photos. However the GIF format is better at retaining sharp lines and can be used effectively for logos and has a simple transparency feature. A related format (sometimes called GIF89a) is an animated form of the GIF image and is commonly used on the web in the form of banners.

PNG is a more modern format similar to the GIF that is gaining popularity on the web.

There are patent issues with the format due to the compression technology used to create the images and commercial online companies are expected to purchase a license. Wikipedia has more details about these issues: GIF.