The Linux man-pages project documents the system calls provided by the Linux kernel and the library functions provided by the Standard C library (with particular focus on glibc, the GNU C library) in Sections 2 and 3 of the manual, respectively. The man-pages project also provides some of the man pages you'll find in Sections: 4, devices; 5, file formats; and 7, overviews, conventions, and miscellaneous. (Many of the man pages on a Linux system, especially those in sections other than 2 and 3, come from other packages.)
You can download the latest man-pages tarball here; older tarballs can be found here.
You can browse the manual pages online here.
If you find a bug in a man page, then please report it.
If you want to contribute to work on man-pages, have a look here.
For a list of changes in various man-pages releases, see the changelog.
For information about translations of man-pages, take a look here.
The man-pages maintainer is Michael Kerrisk (mtk in the changelog; mtk.manpages-AT-gmail.com; blog).