I've always been curious too.
Ping List: Keeping a list of people, for example in a text file, that you put in the TO: portion of a post. When the person checks 'My Comments' section the remark you posted shows up to them. I use ping lists to make people aware of articles I think they may be interested in.
BookMarking: When you click on BookMark in an article it saves a link to it on your profile page. To see your profile page click on you name, you can also click on the names of other people, like me, and see their profile and what they have bookmarked.
