It depends what filesystem you are using.
ext2, reserves some space for the root user, this happens automatically. You can use some options when making the filesystems so that it does not. However it is need on most partitions so that if a another user fills it up you can still log on as root to clear things out.
ext3, same as ext2 but also the journal takes up space.
rieserfs, i think it reserves some space (can somebody else clarify this please) and also for the journal.
There are other filesystems out there but I haven't played with them yet.