The root directory is /.

The home directory is /home/user.

The default home directory is /home/user (where "user" is the username), sometimes shortened to "~".

"cd" with no directory takes you to your home (login) directory. In a path, ~ (tilde) means your home directory, ~usr means the home directory of user usr. For example, "cp ~/foo ~john" copies file foo from your home directory to john's.

The root directory is the top level directory of the entire file system. Every branch starts from there.

The current working directory is where you happen to be in the tree at the moment. If the root is always "/" and my process is in the directory /usr/local/bin/test/data, then the root directory is still "/" and my working directory is currently /usr/local/bin/test/data

It is the Home Directory.