You should be aware of a few key factors...
There are also different colour depths (palettes): Indexed color and Direct color.
- Indexed means that the image can only store a limited number of colours (usually 256), controlled by the author, in something called a Color Map
- Direct means that you can store many thousands of colours that have not been directly chosen by the author
PNG-8 - Lossless / Indexed
PNG-8 can only store 256 colours, like GIFs, it is really a good replacement for GIFs because (although animation isn't as supported), it adds Alpha Transparency (see below for a comparison).
PNG-24 - Lossless / Direct
PNG-24 is a great format that combines Lossless encoding with Direct color (ie. thousands of colours, just like JPEG). Unfortunately PNG-24 files will still be bigger than JPEGs (for photos), and GIFs/PNG-8s (for logos and graphics), so you still need to consider if you really want to use one.
Even though PNG-24s allow thousands of colours while having compression, they are not intended to replace JPEG images. A photograph saved as a PNG-24 will likely be at least 5 times larger than a equivalent JPEG image, with very little improvement in visible quality. (Of course, this may be a desirable outcome if you're not concerned about filesize, and want to get the best quality image you can.)
Just like PNG-8, PNG-24 supports alpha-transparency, too.
I hope that helps!