Concourse is a CI system composed of simple tools and ideas. It can express entire pipelines, integrating with arbitrary resources, or it can be used to execute one-off tasks, either locally or in another CI system. So far, it has largely been developed by Cloud Foundry.
Resources are to Concourse as plugins are to Jenkins. In other words, resources allow Concourse CI to do just about any work necessary in a build. But resources follow a "service provider interface" that makes them easy to build in any language (not just JVM languages) and have a clearly defined computing model, built for composition. Resources don't clutter UI or tax performance.
Con Limited infrastructure options
The downside of building on BOSH is that a full, scalable deployment of Concourse CI requires AWS, vSphere, or OpenStack. If you don't already have these, any one of them can be a big effort to set up, just to get a build server running. Might not be a good fit for smaller teams.