Interlaced GIFs

Interlacing an image allows the viewer to see what it looks like before it actually finishes loading on the web page. You interlace an image when you save it as a GIF file in your image-editing software. You may be given the choice to save the image as "Interlaced" or "Normal" (non-interlaced).

Detailed explanation from the Yale C/AIM Web Style Guide

Interlaced GIF example
Non-interlaced GIF example

Do-It-Yourself Guide

Graphics