Is there something like a "hidden mode"? When somebody else uses my laptop, he can of course see all the windows I have currently open. I'm looking for a mode, which allows me to completely hide some windows but without closing them.

This could be useful if you have your private email and Facebook always opened in a browser window, but you don't want that coworkers could accidentally or intentionally open them when they do something at your laptop.

I'm not thinking about situations when somebody really wants to spy. I think about situations when - for example - a coworker explains you something at your laptop and you don't want him to accidentally see your private mails when he - for example - switches into the browser.
You could use something like GhostTile for this. It is paid app though. It allows you hide applications from Dock, Cmd+Tab, Force Quit Applications, etc. You can configure system-wide shortcut key to make the hidden apps visible.