Want to see what Google knows about you? This video (2 minutes 38 seconds) will show you how to check what sort of information is being archived, and the privacy controls you can use.
This isn't totally comprehensive and is certainly not representative of your whole digital footprint, but it gives interesting insight into how information as gathered and tracked. This is a great starting point to discuss privacy and online activity with students. It clearly illustrates that no action goes unarchived on the Web.