Instagram is a free social networking service built around sharing photos and videos. It launched in October 2010 on iPhone first, and became available on Android in April 2012. Facebook purchased the service in April 2012 and has owned it since.
Like most social media apps, Instagram allows you to follow users that you're interested in. This creates a feed on your homepage, showing recent posts from everyone you follow. You can like posts, comment on them, and share them with other people.
In addition to posting regular photos and videos, which stay on your page permanently, Instagram also supports stories. If you've used Snapchat, or most other social media platforms, you'll be familiar with these. Stories allow you to post many photos and video clips in a series. Anyone can view these for 24 hours, after which they expire.