There is this options box shown when saving a PNG image from Adobe Photoshop. I always choose 'None'.
What does the 'Interlaced' option do?
Interlaced image loads an early degraded version of the whole image as soon as possible and then progressively renders the image to clear state.
Non-interlaced image will load up in tiles showing clear image in each tile as it progresses to load in the image.
"Interlacing" means that it draws (I'm pulling numbers out of the air) every fifth line (line 5, 10, 15), then every fourth line (line 4, 9, 14), then every third line, etc. until the image is filled in, rather than drawing line 1, 2, 3, 4, 5, etc. in order. This allows a sketchy version of the image to come in gradually and fill in until it's completed. Drawing the lines in order means you get the image from the top down.
Lollero's visual is an excellent demonstration.