This is a usual doubt before registering a domain. Will try to explain in simple terms :
1) Domain registrant: Any person who wants to register a domain name is called as domain registrant. That means, an end customer.
[When you decide to buy a domain, you reach out to domain registrars or resellers.]
2) Domain registrar: Registrars are accredited organizations that take responsibility for processing the registration of domain names.
3) Domain resellers: Reseller registers a domain on behalf of registrants, but they don't have a contractual relationship with ICANN.