The "other" file permissions on Mac OS X

OS X, being a BSD-derived Unix, has file permissions which supersede the Unix file permissions you may be familiar with. These are called flags and only apply to files. They can be applied by using chflags <flag> file and removed with chflags no<flag> file (e.g. chflags uchg file makes file immutable). Do man chflags for a list of flags. To see what flags are on a file do ls -lO for example, ls -lO $HOME | grep Library will probably have the "hidden" flag set.