In relation to the internet, a Registrar is an organization or company that manages the reservation and registration of domain names on behalf of individuals, businesses and other entities that would like to use that domain name.
Registrars follow domain registration laws, privacy policies, data security and consumer rights. They have to adhere to all of the relevant laws and contracts with the domain registries as per their accreditation to work legally.
Some of the popular domain Registrars are GoDaddy, Namecheap, Google domains, etc.
A general rule of thumb in the selection of a Registrar is its pricing, the quality of customer support it provides, how the interface both to users and to registrars looks like (an obvious one but important), and any other extra service it provides, and also to check its reputation within the industry.