Notifications are very useful if you need to know when a change has been made to a Google spreadsheet you own or are collaborating on.
Go to the Tools menu and select Notification rules. You can choose to be notified when; any changes are made or when a user submits a form. When changes are made you will receive an email which will show you the comment, giving you the option to open the comment in the spreadsheet or you can simply reply to the comment straight from your email without having to be redirected to the spreadsheet. Also if your collaborators have edit access you will be informed by email when a change is made with a link taking you to the highlighted edit on your spreadsheet. This makes changes much easier to find and track!
You decide if you want to be notified straight away, or if you already have enough emails to sieve through, you can limit notifications to a daily digest – very handy!