What is the difference between a ramjet and scramjet?

In a scramjet, the combustion chamber is designed in such a manner that the airflow is supersonic, whereas in the combustion chamber of ramjet, the airflow is subsonic. You can read about the Hypersonic Technology Demonstrator Vehicle (HSTDV) – unmanned scramjet technology of DRDO in the given link.

Further readings:

1. Advanced Technology Vehicle – India, one of the 4 Countries To Develop Scramjet Engine
2. Reusable Launch Vehicle – Technology Demonstrator (RLV – TD)