What is the difference between RD and REQM?

Requirements Management, or "REQM," is all about maintaining the set of requirements that you have, and the process of accepting new ones. In the CMMI world, that includes understanding them, committing to them, managing changes to them, maintaining the appropriate degree of traceability, and understanding how they relate to the actual work being performed by the team.

Requirement Development, or "RD," is about the transformation of customer needs into requirements that can then evolve into a design and/or code. This includes eliciting the customer needs (JAD sessions, interviews, et al), transforming those needs into requirements, evolving them into product requirements, allocating the requirements across releases, teams, developers, or modules, validating them, and ensuring that they fit within the customer constraints and assumptions.

I am not aware of one single tool that does all of this, but parts can be supported by Borland's CaliberRM, IBM's ReqPro, or Doors. My preference is CaliberRM, but they all pretty much do the same thing. They are also all parts of a larger "development framework" that these companies market.