In keyboard shortcut settings I see a launcher section that have 'WWW' and 'Explorer' labels inside the key combination column of the shortcut definition. What keys I must press to launch my browser and my file explorer in Ubuntu 12.04 if I want use the default system shortcuts.?
If you want to create a shortcut for opening Nautilus or your browser, open system settings, go to "Keyboard" and switch to the "Shortcuts" tab.
Select "Custom Shortcuts" and click the "Plus" (+) button, give it a name and put 'nautilus' in the command field. Then click "Apply".
In this case you will see a new shortcut called "Open Nautilus" that is disabled. Click on "Disabled" and enter the shortcut you want.
This will work for your browser as well, you just have to know its command.