What is inode number?

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An inode is a data structure on a traditional Unix-style file system such as UFS or ext3 / ext4.
An inode stores basic information about a regular file, directory, or other file system object.
iNode number also called as index number, it consists 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)

Following command will be used to show inodes of file and folders:
ls -i
Following command will show complete info about any file or folders with inode number
stat file/folder