Virtually all municipal bonds are now issued in registered form: a list of owners is kept and updated each time a bond is sold or transferred. The registrar is the company that keeps track of bondholders for the issuer.
The registrar works closely with the paying agent, the entity responsible for making interest and principal payments to bondholders. In practice, the entity acting as registrar may serve in several roles for the issuer – the bond trustee may serve as registrar, paying agent, and transfer agent.