What is Pinterest?
Pinterest is an image sharing site aimed at women to pin their favourite images for all sorts of topics to boards, a bit like creating a vision board and really popular for planning a Wedding, a renovation and more. It has a lot of 'how tos' for DIYers and arty women.
Should you be using it in your business?
If you market to women in any way and have images you can share, then yes most definitely set up an account and start sharing images of products and images that would appeal to your audience and that are relevant to your product or use it to show what your business is in to and all about. If you are wanting to become known for something in particular perhaps you can post some 'how to' instructionals or video.

To look professional, trustworthy and legitimate in your business it is best not to use free services for email or web hosting for that matter, domains and email hosting are really affordable and just makes you look that much more professional. You will potentially outshine your competition with this one small thing and yes it is small, but people do notice.