How to develop in Dynamics CRM?

At my organization, I'm working on a cloud based application that delivers information to sales team in the Dynamics CRM. It's required to build a UI in Dynamics CRM or integrate a UI with Dynamics.

Is this possible? If so, can anyone link me to any good resources on the matter or provide me with any information you feel would be relevant? I've been doing a lot of research but haven't found anything about how to develop in Dynamics (programming languages, development framework for both UI (native vs HTML/hybrid) and backend changes).

Thanks a lot.