A registrar (a domain names registration office) is a private company in charge of administrative and technical actions of a domain name’s registration with the concerned registries.
It acts as intermediary between the « Registrant » (Applicant) and the « Registry». It is registered with different domain names registries depending on the extensions it wishes to commercialize.
THE REGISTRAR’S ROLE
The registrar is responsible for the database maintenance of the domain names reserved with it, and the database’s update of registries which it represents.
The DNS system functioning depends on these updates.
A unique Registrar is recommended to facilitate the domain names’ portfolio management of an entity. The chosen registrar must be known for its reliability and professionalism and it must be accredited.
Source: Nameshield’s White paper – Understanding domain names