Google collects data on you via tracking cookies, plugins and a set of applications such as chrome browser, search page, Youtube, maps, playstore, drive, gmail and agenda. The quality and accuracy of the data can be very high.
Expect all your searches, a history of locations, watched or liked video’s on youtube and your agenda. The data is supplied via an email (which can take a day) and you can expand the data on your hard drive – note that this can be a very large file (1GB).
Some of the files are so large that you may need tools like Atom to browse it.