Google stores and tracks every single search term you type. Same goes with Big Social Networks such as Facebook, Twitter, etc. based on articles you look at or messages you post, etc.. They then build profiles on you based on your history and attach socioeconomic tags to you based on which tags they follow you around with
Ads, to influence what you think, what you buy, who you vote for, etc.. And based on which traces you can end up paying more for a flight, more for a hotel, not shown a job post, etc.. And this does not stop at what their partner/client merchants offer you, it can even extend to how Governments with access to this data treat you.