SQL & PL/SQL

Hello,
I'm searching for a possibility to find out which users are connected to my DB. Is there a table which keeps
eye on all connections like in MS-SQL-Server?

It would be great if it is possible to disconnect all users from a DB by script / stored procedure so that I can
restore the database afterwards.

I don't want to kill all users connected to the Oracle instance, as there are many developers each connected to his own DB on the same instance.
Therefore I can't just shut down and restart Oracle.

Thanks in advance,
Salek Talangi