Instagram is photo sharing (and now short videos) and is mostly used via and app on a smartphone. You take a photo, add some text and post it for your followers to see. It is similar to twitter but every post is an image. Instagram is now owned by Facebook.
Instagram users are young, 90% of the 300 Million users are under 35, usage has doubled in the last two years and overtaken Twitter.
Instagram is all about the hashtags. Pictures have short snippets of text associated with them that include mostly hashtags like #sunset #pizza etc. This is how people find photos and people to follow as photos with the same hashtags can be found by searching. When you create an instagram profile make sure you include a link to your website. Regular Instagram posting is a good way to build followers.
You cannot add links to individual posts to promote your website. Followers have to click through to your profile to find a link to your website.