Basically what it is is a forum where you can 'pin' photos to 'boards' that you create. The boards are like moodboards or idea boards.
Basically and very simply what you do is look through various categories and click on whatever interests you to get started, then create boards, and pin any pins that interest you, using the pin it button. There is a search function too. A search for strawberry ice cream, for instance, will yield many photos and recipes for strawberry ice cream from many different accounts. You can then pin away to your heart's content.
You can 'follow' someone's board or someone's entire account, and when they pin something it appears in your home feed. Likewise if someone follows you, your newest pins appear in theirs. DD3, DD4 and I follow each others' accounts and we comment on pins we all pin occasionally. It's a nice way to be able to keep up with each others' interests. The DDs tease me about all the recipes I have collected ('less collecting and more cooking please').
When you pin a pin you can edit the comment under it to give a better reflection of what it is. This can affect what search terms will pick it up. You can also comment on someone else's pin in a comment box and the pinner can read your comment and respond if they like.
On my Pinterest account I have I think about 45 boards so far including recipes for dinner, baking, desserts, a fashion board or two, boards on Dublin, the Baltic, Istanbul, all things Japanese, Scandinavian Christmas style and Scandinavian cuisine, a handy household tips board, one on embroidery and another on sewing/knitting/crocheting, one on gardening ideas, various artsy boards featuring arts I am interested in propaganda posters, photographs I like, book illustrations and one on tea paraphernalia, plus interior design, a board devoted to cakes, and a few history boards the Normans, etc. I also have a nerdy board featuring pins people think are Irish scenes but are internet and Pinterest hoaxes. (People seem ready to believe just about anything fanciful when it comes to what Ireland looks like).
I also have a board on vintage cars that unfortunately attracts the interest of some men whose boards feature images that are quite porny so beware of letting your DD set up any board that might attract the attention of men who like cars, motorbikes, tattoos, guns, crossbows, hunting, fishing, Japanese schoolgirls, lingerie, and other topics that could conceivably interest that sort of man. I would say from my own experience that one in ten of the male users of Pinterest have boards of that sort, sadly. However, anything objectionable you come across can be flagged and reported, and I have done just that when I come across material that I object to.
You can also block other specific pinners' access to your account.
I know making girls modify expressing what interests them in order to escape the interest of men who objectify women is a topsy turvy way to approach the problem, but I would prefer not to have the DDs exposed to that all the same.
DD4's interests lie in evening wear, jewellery, and shoes that are way too high, the Tudor period, American colonial days, Harry Potter and Paris, and she also has a board featuring cats, kittens, dogs and puppies and funny animal pictures.
DD3 likes fashion and animals, and also science, optical illusions and travel, and she has a humour board.