What is the difference between datasource plugin and backend plugin?

I’ve released two plugins for Riverbed products, these plugins come from from 'grafana-simple-json-datasource
They work witha Python Flask-server but I need to get rid of Python.
I’m wondering what goal is backend-server plugin is about, I do not cleary understand it’s definition in Grafana docs (my english is pretty poor).
Can someone help me ?