Green, it's an online pinboard. When you view a website, you can pin a copy of an image to your board. By clicking on the image, you visit the site the picture originates from.
So, I can read an interesting tutorial, pin apicture from it to my pinterest board, and it's waiting for me, like a bookmark. You can see other people's boards, too, unless they are private, and can repin something from their board to yours.
ClaireDeTamble - It is copyright infringement because you are using an image from someones site without their permission and saving it as a new image file as if you are the originator of the image.
Alternatively, if not saving an image as a new image file then people are hotlinking to the original image, which is in fact theft of the original sites broadband that they obviously pay for.
Anyway the easy way to make sure you don't infringe anyone's copyright is to 1/ Avoid any images that have watermark copyright logos on them 2/ email owner of the image and ask their permission to use it
Anyway as far as I can tell individuals who pin copyright images will just have the image removed from their pinboards by Pinterest or if a persistent offender have their boards deleted.