Eris cli is basically what Eris is - github.com 
The cli's main goal is to do service management: spin of containers with chains and other such required services.
Best way to use eris cli is through Docker, which will help setting up and running eris-db (and individual chains) through docker.
The simplest way to get started is by simply running
$ erisdb
That will start a fresh node with
~/.erisdb
as the working directory, and the default settings. You will be asked to type in a hostname, which could be anything. anonymous is a commonly used one.
Once the server has started, it will begin syncing up with the network. At that point you may begin using it. The preferred way is through a Eris Javascript API, but it is possible to connect directly via HTTP or websocket. The JSON-RPC and web-api reference can be found here.