Root directory:

All files and directories are considering the root directory .because the root directory not contained 

for anything from top and bottom. 

Sub-directory:

Anything contained for inside the directory .so its called as a sub-directory.

Change Directory:

1. "Cd ":

This command is used for moving your current working directory or user's home directory.

"/":

Root directory:

The root directory is always represented symbols like as a ,  "/". 

The root directory is change into your current working directory:

Cd  /

Because the root directory in main importance for everyone stream.then other directories are defined by below.

Cd..

To a directory  one level up from a current working directory.

Moving scenario like as a,

Parent directory  _  current working directory.

Because its take one step to a back . meaning of contents like is a parent to current working directory.

Cd.. will switch into parent of your current working directory.

Tilde command(~):

Its referring for your user's home directory.

its the user name is username and home directory is home

Cd ~

This functionality is put into the same like as a,

Cd/ h/ home/username

Otherwise it put like as a,

Cd ~/documents

Cd - :

When this command is used to moving your previous working directory

For example like as a,

Cd ~

pwd

/user/home

Cd documents/ economy

pwd

Cd -

This command is reach on previous working directory.