Pinterest can be a lot of things to a lot of people, and it's just barely catching on for automotive enthusiasts.
You can create your own "boards" which are basically folders and label them as you wish. You then "pin" things to the boards.
It's a great way to catalog everything you ever SEE and want to remember. If you see a user who has similar interests to yours, you can "follow" that user, get notified when they "pin" things and then in turn, pin them to your boards where applicable if you want to remember them too.
It can be used for many other things too.
You can see one example at this site, but this site is more of a storefront. Pinterest is MUCH more than that, so it's worth it to look deeper to see what it's really about.