Reddit is using browser fingerprints to identify and track you, even if you're not logged in. And if that info is shared, it can be used to track what websites you visit. Browser fingerprints work somethig similar to this: let's say you go to a website, the website needs to display correctly, so it gets info like, what browser you're using, what version of java or plugins etc...you have to display accurately. Your specific browser version, what update it's on, what plugins and extentions you use, windows version or mac, etc...when all combined can be fairly unique to your specific computer to the point where it can be used to identify you, at least for a while until you update/change your setup.

edit: somehow I missed the second part of your sentence and thought you were asking something else.