JPG nor PNG for web

Lately I have noticed a lot of PNG files on the web.  I use JPG.  What is the difference?

I use jpg when small file size is important, png when high quality and/or transparency are needed. Also, when converting vector graphics for the web, png allows me to actually pick the colors I want to delete to reduce file size, which can actually result in a smaller file size than a jpg for the same graphic.

Tiff files for printing (4c offset, newspaper, magazine, etc.) turn out better when created from a png file than from a jpg. I constantly get submissions in both formats, that's how I know.

In general, the png file is the better of the two, it's just not always necessary to accept the file size penalty, so jpg has its legitimate space as well.