You can post almost anything that can be represented with activity streams -- short or long text, bookmarks, images, video, audio, events, geo checkins. You can follow friends, create lists of people, and so on.
There's an API defined in the API.md file. It uses activitystrea.ms JSON as the main data and command format.
The software is useful for at least these scenarios: - Mobile-first social networking - Activity stream functionality for an existing app - Experimenting with social software