What is difference between these command?

The simple command cd <dir> which changes directory to <dir>.

• ~ indicates $HOME directory
• / indicates the root directory
• ~/ indicates the $HOME directory as well. The only difference is that it explicitly shows it's a directory (the trailing slash). cd ~/ and cd and cd ~ and cd $HOME all do exactly the same thing.
• cd - Changes the working directory to the previous working directory.

These special symbols "." (dot) and ".." (dot dot)[Relative Parameters]:

The "." symbol refers to the current directory and the ".." symbol refers to the current directory's parent directory.

$USER and $HOME are Environment-Variables

$USER = The name of the currently logged-in user. This variable is set by the system. You probably shouldn't change its value manually. (ex:myuser1)

$HOME = The location of the currently logged-in user's home directory.(ex: /home/myuser1)

Recommended to use cd "$HOME" or cd "$USER" so-that cd gets proper input in case of space, etc.

cd ~

Changes to your home directory. ~ at the beginning of a path is an abbreviation meaning “the user's home directory”.

cd /

Changes to the root directory /. Nothing special going on here.

cd ~/

The trailing / doesn't make any difference. It forces ~ to be interpreted as a directory, but cd does that anyway. (A trailing / makes a difference on a symbolic link to a directory — compare ls -ld /var/spool/mail and ls -ld /var/spool/mail/.)

cd -

Changes to the directory that you were in before the previous cd command. This is a special case of the cd command: when its argument is -, it does this.

cd --

With most commands, including cd, the argument -- means that anything that comes afterwards will be treated as an operand rather than an option. So for example cd -- -P means to change into a directory called -P, whereas cd -P passes the -P option (which makes a difference if the path that you change into goes via a symbolic link). When there is no argument after --, the -- doesn't do anything; this command is equivalent to plain cd. cd with no argument, in turn, is a shortcut for cd ~.

cd /.

Equivalent to cd /, since /. is the same directory as / (. is mostly useful on its own, to mean “the current directory”).

cd $HOME

Changes to your home directory. This fails if the path to your home directory contains a space or other characters. Always use double quotes around variable substitutions: cd "$HOME".

cd $USR

In all likelihood, this does nothing because no variable named USR is defined in your shell, hence the command that runs is just cd.