Muddiest Point for 10/20 Lecture

During this Tuesday's lecture, we briefly discussed the use of cookies on websites. I've always wondered if cookies are dangerous? Do they have the ability to access personal information? When we talked about them in class, they appeared to be harmless, as a way to track returning visitors to a website. However, whenever I run an ad scan, it usually removes the cookies from my computer. So are they harmful or simply a way to track returning visitors to a website?