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The conventional (non-interlaced) GIF graphic downloads one line of pixels at a
time, and Web viewers like Netscape display each line of the image as it
gradually builds on the screen. In interlaced GIF files the image data is stored
in a format that allows Netscape (and other viewers that support interlaced GIFs)
to begin to build a low-resolution version of the full-sized GIF picture on the
screen while the file is still downloading. The "fuzzy-to-sharp"
animated effect of interlacing is visually appealing, but the most important
benefit of interlacing is that it gives the reader a quick preview of the full
area of the picture. This preview effect can be misleading 
interlaced graphics are not faster-loading than non-interlaced graphics,
they just look as if they download faster because the rough preview comes up
faster.
Note that the examples below only work well the first time you try them.
After that your browser will probably cache the images locally, and subsequent
loading will occur (very quickly) from your hard disk, not from the Web. Use
your browser's "reload" button to repeat the loading of the graphics
if they load too quickly to see the difference. Both example photographs are
GIFs, identical except for interlacing.
Example
of an interlaced GIF graphic.
Example
of a noninterlaced GIF graphic.
References
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