From the about page:
Pinterest is a place to discover ideas for all your projects and interests, hand-picked by people like you.
From Wikipedia (retrieved on June 8, 2015):
Pinterest is a free website that requires registration to use. Users can upload, save, sort, and manage images—known as pins—and other media content (e.g., videos and images) through collections known as pinboards. Pinterest acts as a personalized media platform. Users can browse the content of others on the main page. Users can then save individual pins to one of their own boards using the “Pin It” button, with Pinboards typically organized by a central topic or theme. Users can personalize their experience with Pinterest by pinning items, creating boards, and interacting with other members. By doing so, the users “pin feed” displays unique, personalized results.
Pinterest is headquartered in San Francisco, CA (USA).