Quickly change to an open application

Sometimes it's difficult to return to a file or an application that's open by clicking an open window in the usual way. Here are two ways:

In System Preferences, set one of the corners of your desktop so that when you move the pointer into it it shows you either Mission Control - all the windows and applications that are open - or just the applications.

Or, press cmd-tab. You can then tab across the row of your open applications and stop on the one you want to return to. Neat and quick.