Hi, first, the easy part...
To stop any application in MAC, the easy way it would be to CLICK on the ICON for 2 seconds
and then choose the QUIT button
If your computer has multiple user accounts, switching between users can be inconvenient. Normally, to allow another user to log in, you have to log out of your account completely, closing all your documents and quitting all open applications.
However, you can allow more than one user to say logged in to the computer at the same time by turning on “fast user switching.” When fast user switching is turned on, another user can log in and use the computer while the previous user stays logged in and all their applications and documents remain open. When the second user is finished using the computer, you can easily switch back to the previous account.
To turn on fast user switching:
Choose Apple menu > System Preferences and click Accounts.
Click the lock icon to unlock it, and then enter an administrator name and password.
Click Login Options.
Select “Show fast user switching menu as”, and choose how you want the user’s to appear in the pop-up menu
The current user will appear in the menu bar in the upper-right corner of the screen.
To quickly switch between user accounts:
From the name of the current user, in the menu bar in the upper-right corner of the screen, choose the account you want to log in to.
Hope this helps!