inode in linux/unix filesystem

inode:
An inode is a data structure on a traditional Unix-style file system such as UFS or ext3. An inode stores basic information about a regular file, directory, or other file system object.
The inode (index node) is a fundamental concept in the Linux and UNIX filesystem. Each object in the filesystem is represented by an inode.
Each and every file under Linux (and UNIX) has following attributes:

(*) File type (executable, block special etc)
(*) Permissions (read, write etc)
(*) Owner
(*) Group
(*) File Size
(*) File access, change and modification time (remember UNIX or Linux never stores file creation time, this is favorite question asked in UNIX/Linux sys admin job interview)
(*) File deletion time
(*) Number of links (soft/hard)
(*) Extended attribute such as append only or no one can delete file including root user (immutability)
(*) Access Control List (ACLs)

All the above information stored in an inode. In short the inode identifies the file and its attributes (as above) . Each inode is identified by a unique inode number within the file system. Inode is also know as index number.