Every object on a filesystem is assigned a unique index, called an inode number. This might seem trivial, but understanding inodes is essential to understanding many filesystem operations. For example, consider the . and .. links that appear in every directory. To fully understand what a .. directory actually is, we'll first take a look at /usr/local's inode number:

$ ls -id /usr/local
   5120 /usr/local

The /usr/local directory has an inode number of 5120. Now, let's take a look at the inode number of /usr/local/bin/..:

$ ls -id /usr/local/bin/..
   5120 /usr/local/bin/..