Representational State Transfer (REST) is the protocol used to communicate with the APIC-EM server. Any programming language that supports HTTP/HTTPS calls can be used to make REST calls.
API reference come with the installation package. Once you have APIC-EM running you can access reference doc form UI. You can also view the reference doc from DevNet- https://developer.cisco.com/site/apic-em/documents/api-reference/
There is a pyhton SDK for APIC but not aware of any for APIC-EM.