zlib and gzip?

I'm totally confused now.. Can somebody tell/show me the difference between gzip and zlib encoding, as they are in php.ini? First i thought gzip and zlib where two complete different compression libraries, but it seems they're from the same, both gzip. Then i don't see the difference anymore in using them in php, though php.ini still gives me the impression they're totally different.. like clouds in dark woods.. M. Vek