If you want to select your preferred applications for Web browser,Mail reader and terminal emulator you can select very easily from gnome preferred application option.
For this you need to go to System—>Preferences—>Preferred Applications you can see this in the folloqing screen
Once it opens the window you should see the following screen here you can select web browser and mail reader applications this is very helpful if you have 2 or 3 application which does the same job and you want to use one of your favourite application as preferred application.
If you click on system tab you can select terminal emulator
