1. A data structure holding information about files in a Unix file system. There is an inode for each file and a file is uniquely identified by the file system on which it resides and its inode number on that system. Each inode contains the following information: the device where the inode resides, locking information, mode and type of file, the number of links to the file, the owner's user and group ids, the number of bytes in the file, access and modification times, the time the inode itself was last modified and the addresses of the file's blocks on disk. From Linux Guide @FirstLinux

2. An inode is the address of a disk block. When you see the inode information through ls, ls prints the address of the first block in the file. You can use this information to tell if two files are really the same file with different names (links). A file has several components: a name, contents, and administrative information such as permissions and modification times. The administrative information is stored in the inode (over the years, the hyphen fell out of "i-node"), along with essential system data such as how long it is, where on the disc the contents of the file are stored, and so on. There are three times in the inode: the time that the contents of the file were last modified (written); the time that the file was last used (read or executed); and the time that the inode itself was last changed, for example to set the permissions. Altering the contents of the file does not affect its usage time and changing the permissions affects only the inode change time. It is important to understand inodes, not only to appreciate the options on ls, but because in a strong sense the inodes are the files. All the directory hierarchy does is provide convenient names for files. The system's internal name for the file is its i-number: the number of the inode holding the file's information. From UNIX Progamming Environment