Application to switch among multiple running applications
Case #1: You can get back to recently used apps by holding down the home key until a menu appears, and then selecting the app you want to return to.

Case #2: Press the home button and open your dictionary app, and then use the method in #1 to return to messaging.

This works if you don't have too many apps open, since the memory is limited and if you open too many apps, android will close them from the background.

The important distinction is that the home key does not quit the app but exists to the launcher, and the back button exits the app.