Google Analytic’s Annotations allow users to leave little notes attached to specific dates. These notes are then display attached to that date in your report graphics in Google Analytic’s. By double double clicking on any of the Circles in a graph you open the Create new Annotation section.
You can save the Annotation so that only you can see it or you can save it so that its Shared and anyone with Access to this Google Analytic’s account can see it. Which is very nice you can add Annotations to days you release a new version of a product, when your company posts some news or you make a new blog post. It makes it easier to see when something special happened and what kind of results it may have had in the different reports. An annotation is created on a date so even if you create it in one report its still there in all of the other reports.
As you can see there was a spike in the number of hits on this site on Oct 22. I added an Annotation for it. Now all the users you can see that there is an Annotation for that date by the little Note icon on the bottom. They can then read my description as to what happened on this date.