GIF files are a format commonly used for graphics presented on websites. GIFs can contain a maximum of 256 colors, and are therefore best for images that contain simple shapes, a limited color palette, text and other elements as opposed to photos. GIF stands for Graphics Interchange Format.
GIF files have the file extension .gif. GIF files are compressed using "lossless" compression, meaning image quality is not sacrificed as file size reduces. However, if you save images with many colors as GIF files with a limited color palette, you will notice a reduction in quality. This is most obvious in photos.
Unlike JPG files, GIF files support transparent backgrounds. This allows GIF files to blend with website background colors. However, since pixels can only be 100% transparent or 100% opaque, you cannot use them for partial transparency, drop shadows, and similar effects. To achieve that, PNG files are best.
GIF files can also contain animation, creating files known as animated GIFs. These are commonly seen on websites, though they are not as widely used as they used to be. Remember the days of animated "under construction" graphics? Those were classic animated GIFs.