One other thing about cookies is that some browsers allow you to enter a 'privacy' mode. So if you use privacy mode when looking for birthday presents, holiday destinations, etc, the next person using the computer won't get ads related to what you were doing (or at least there's less chance), so long as you close the browser when you finish.
(closing the browser means any temporary information, such as the cookies, can be deleted and then next time anyone browses, there's little to show you'd ever seen the website(s) before.)
Firefox calls it "Private Browsing" and when you switch to it, it closes the windows/ tabs currently open to do the "switch".
Similarly, Safari "switches" from "ordinary" to "Private Browsing".
Chrome can provide "ordinary" and "incognito" browsing at the same time.
Here's some text displayed by Chrome:
Pages you view in incognito tabs won’t stick around in your browser’s history, cookie store, or search history after you’ve closed all of your incognito tabs. Any files you download or bookmarks you create will be kept.
There's a web link for more information, and then it says:
Going incognito doesn’t hide your browsing from your employer, your internet service provider, or the websites you visit.
I suspect that's there to remind browser users that lots of things can be done to track the sites used by anyone, not all of them under "local" control (such as the browser).