Imagine you have created a Google Sheet that a number of people have the rights necessary to edit. It may be useful that when the spreadsheet is edited - you get notified by email. This is easily set up. While you are viewing the spreadsheet from which you would like notifications - click on Tools > Notification rules > and then select the type of notification to require.
It is also possible to be notified when somebody completes a Google Form that you have created. It works in a very similar way. While you are looking at the spreadsheet of responses - follow the same steps as above.