NTFS
NTFS is the native filesystem of WindowsNT. it supports security features in the filesystem such as access control. Since WindowsNT is entirly unicode, NTFS is a unicode filesystem, each "character" being 16bits wide.
About NTFS (New Technology Filesystem)
NTFS adds quite a bit more to HPFS than just security features, though. First, it adds quite a bit of builtin redundancy -- with HPFS, wiping out one sector in the wrong place can render an entire volume inaccessible. Second, it adds support for multiple hard-links to a file (up 'til now, the only easy access has been via the POSIX subsystem, but NT 5/Win2K adds this to Win32 as well). Third, it supports an arbitrary number of file forks a la MacOS (except MacOS always has exactly 2 forks per file). Fourth, HPFS decrees that a cluster is always 512 bytes, and a cluster is always one sector. For the sake of performance and compatibility with some (especially Japanese) machines, NTFS allows sectors of other sizes. It also supports clusters of more than one sector, which tends to help performance a little.
NTFS is probably one of the most difficult file system to deal with, especially because of the lack of hacking experience and reliable documents about it. A read-only stable driver is in Linux source code base since kernel 2.4, while an experimental read-write driver is coming with linux 2.6.
The NTFS-3G project apparently has a Read/Write implementation for Linux/FreeBSD/BeOS that is considered by some as adequate.