

Well...PNG is the sucessor to gif. gif was used for it's animation capabilities and transparencies, but it supported an absurdly low 8-bit of colors (yikes). PNG can handle transparency, animation, and displays images in true-color (16 million I think?)
JPEG is popular because of its compression - try setting your digital camera to save pics as .tiff and see how many fewer pics your camera can hold :O
(sorry, i had a basic art class two(?) semesters ago and remember having to learn this kind of booooorrrrrriiinnngggg stuff)