Check out some equivalents of popular Windows alt shortcuts on a Mac, which might be especially helpful for those users who are new to Mac.

	Windows	Mac
Closing a window	Alt-F4	Command-W
Restarting	Control-Alt-Delete	Control-Command-Eject
Shutting down	Control-Alt-Delete	Control-Option-Command-Eject
Logging out	Control-Alt-Delete	Shift-Command-Q
Switch between open apps	Alt + Tab	Command-tab
Go back	Alt + Left arrow	Command + Left arrow
Go forward	Alt + Right arrow	Command + Right arrow
Move up one screen	Alt + Page Up	Command + Page Up
Move down one screen	Alt + Page Down	Command + Page Down

References: (i) Apple Support Communities, and (ii) Parallels Desktop Blogs