Irmin is a key-value store based on the same priciples as Git. This means that for existing Git users it provides many familiar features: branching/merging, history and the ability to restore to any previous state.
Typically Irmin is accessed by embedding it into an OCaml application, but there are also several tools like irmin-http, irmin-rpc, irmin-graphql, irmin-resp, which allow you to use it as a standalone server.
It is most often used to store application data, like configuration values, shared state or checkpoint data, but can be used as a general purpose key-value store too. Additionally, since it is compatible with Git, Irmin can be used to interact with Git repositories directly from your application.