Inodes are
not necessarily a feature particular to
unix-like operating systems. They
are a distinctive feature of a particular family of
file systems that is most often used under Unix.
An inode contains basically all the information about file, with one surprising exception: its name. Filenames are defined through directory entries, so the same file can appear in different places in the directory hiearchy, with different names (see hardlink).
Apart from meta information such as access times, the inode in modern filesystems (after the Berkeley FFS) also contains the numbers of the disk blocks that contain the actual file data - or the numbers of disk blocks that contain the numbers of disk blocks where the actual file data is, for larger files (there are also second and third levels of indirection, for really large files). This is important because it means you can access any position within a file with relatively little overhead - something that made the Berkeley FFS much faster than previous file systems.