How to find out what data Google holds on you. What Google knows about you, and what you can do about it.
Update 11 November: Google has launched a new About Me tool that lets you see exactly what information other people can see about you. Plus, any changes you make here are mirrored across all your Google services, including Drive, Photos, Google+ and more. To check it out simply visit https://aboutme.google.com.
What does Google know about me? Your Google Account
In order for Google to put a name to you, you have to have a Google account. Without one Google may well have anonymous data about you as a cookie on a laptop- or smartphone browser, but it is impossible to know what data that is (just as it is impossible for Google to know who you actually are). So all of the processes I am about to explain rely on you being logged in to your Google Account. I bet you have one, too. Ever logged in to an Android phone or tablet, used Gmail, or used YouTube as a registered user?
The email and password you use for those things (or Google Calender, Google+, Google Docs etc) is your Google Account. You may - as I do - have several. In which case you will need to repeat all of what follows for each account. Amuse yourself by spotting the differences between each account.
First, let's check the Basics. (See also: What is OK Google?)
What does Google know about me? Search history and account basics
Google used to be only a search engine. Remember that? Almost certainly you still use Google Search, a lot. And Google collects a lot of data about the searches you make, as it says a lot about you. Sign in and head over to Google and view the search history that Google holds about you. It's actually quite freaky to see what Google knows you searched for. You can also see your YouTube search history here.
More prosaic but also critical basic information can be found in your account details. Sign in and head to https://myaccount.google.com/ to see the basic data you gave to Google. Head to Dashboard and you can see records of an array of Google tools, and your activity with them. We'll click into a couple of these further along, but suffice to say that this is an important dashboard for finding out what Google knows about you. (See also: How to sync Google Calender and Outlook.)
What does Google know about me? What Google thinks it knows about you
This is where it gets interesting. And also where you find the ability to opt-in and -out of interest-based advertising. The principle reason Google tracks all this data about you is so that it can more successfully target you with advertising, in order to pay for the free services you access. Depending on your viewpoint (or demographic) this is either a sinister menace or a cool way of getting free stuff.
But it is interesting to find out just what Google thinks it knows about you, and how it uses that to target ads your way. Sign in and head to Settings. You will see what Google considers to be your gender and age, as well as your interests. Prior to Google+ this used to be great fun (when I was in my early 30s Google thought I was a woman in my 60s). But now if you use Google+ - and you tell the truth - you will find the basic data is largely correct. It is the interests that are - well - interesting.
When I do this test with my work email account Google thinks I am some sort of tech maniac. Which makes sense. But on my personal Gmail accout my interests are naturally more broad. This matters because within this page you have the option to opt out of interest-based advertising. These are adverts targetted at you because Google thinks you will be interested in their content. Personally I am happy to remain in - in part because I earn my living from advertising on this website, but mainly because it amuses me that all the adverts I see at work are for tech.
If you are less happy, as well as opting out of interest-targeted ads, you can also opt out of allowing website owners to track you via Google Analytics. Simply head over to the Google Analytics opt out page.
What does Google know about me? Where Google thinks you have been
One of the options available from the Google Settings Dashboard outlined above, is the ability to find out where Google thinks you have been. You can head there directly at Location History. Interesting to me here is that despite me using an Android phone into which both my Google Accounts are synched, Google has no location data on me. My phone by default doesn't share that data.
What does Google know about me? What devices, apps and services have access to your Google data
In many ways this is the most important test of all. One of the benefits of Android is that it lets you know what permissions are required by each app you install on Android. But you encounter myriad Google apps and services on multiple devices. This page lists the devices and services, and what access they have. Click on a listed device and you can see further details and the opportunity to revoke, on the righthand side. (Also read: Google Calender Tips and Tricks.)