Most Web-capable graphics editors let you save images in an interlaced (progressive) format. Interlacing and progressive formats generally are the same thing—the gradual display of a graphic in a series of passes as the data arrives in the browser. Each additional pass of data creates a clearer view of the image until the complete image is displayed.
The only real advantage to displaying graphics in the interlaced or progressive method is that users immediately see at least a blurred view of the complete image, giving them something to look at while waiting for the entire graphic to download.
The disadvantage of choosing this display method is that older browsers may not display the graphic properly, and more processing power is needed on the user’s machine to render the image. The use of these methods has declined as increased connection speeds become available.