Pinterest is an online pinboard, i.e., kind of like a digital bullentin board. You can set up "boards" for any topic that you choose. I have recipes, my son's upcoming birthday party ideas, clothing, quilting, etc. Then when you are browsing around Pinterest or the internet if you see something that you like you can "pin" it to your board.
I like it because it helps to visually organize things, it keeps my bookmarks from filling up, and since it is online you can access it from anywhere. So, for example, say you are visiting with a friend and you remember that you wanted to show her a new pattern or techinique that you "bookmarked". If you pinned it on Pinterest you would just log into your account and show it to her. With a regular bookmark you couldn't do that you would have to wait until you got home to email her the link. The other thing that is nice about it is there is no more forgetting why in the heck you bookmarked that site because they have moved the site or the link is dead.
Is it another time waster? Yes. However, I think it can be a nice tool if utilized properly. And since quilters are such visual people it just makes sense.