PNGs are lossless - they don't actually throw any information away. JPEGs are lossy - they chuck away pixels they think you won't notice.

However, obviously sometimes you don't want any pixels thrown away at all - usually when dealing with hard edges (like you get with text). In those situations PNGs are definitely better than JPEGs. JPEGs work very well for photos (which tend not to have those hard edges), so if you're trying to save space then JPEGs are good for photos. Otherwise PNG is a better file format most of the time.

(Edit - that wasn't supposed to be lecturing you, just thought some context might help)