UNIX for Dummies Questions & Answers

Hello,

1) I am trying to get involved in UNIX for educational purposes so I have installed the latest Ubuntu edition 12.04. Do you know another package that I could use it instead for educational purposes?

2)What is the difference between "~$" and "/$" (it comes with cd / and cd ~) .The latter "/$" should mean the root user?

3) I also get the root user by using "sudo -s" and I get "root@ubuntu: /#" which I think stands for the root user.I have created some new users with command "adduser".The commands "useradd" and "groupadd" which are morejust don't work (why?).Eventhough I have created the new users when Ι am as "root@ubuntu: /#" Ι can't see the new users? Also when I am connected as "y@ubuntu: /$" ( y is my name) and I type users I only get "y y" who are these users and I why I take them double? Why is that?The same happens when I put "y@ubuntu: ~$".What should I do to see the users?

Thanks for your time...!

1) fine
2) ~: home directory; /: root directory (file system root, not user "root"). $ is the prompt for non-root users ususally used, can be modified.
3) I don't understand. Adding users creates accounts so they can log in. The users command lists users actually logged in (as does who). So y y indicates you are logged in twice, and nobody else.