Registry: Manages the TLD, determining the rules for a Top Level Domain (TLD) (such as .com and .co.uk), and maintains the technical operations necessary to keep the TLD running smoothly.
Registrar: The company accredited by both the Registry and ICANN (Internet Corporation for Assigned Names and Numbers) to sell domains to the public. They act as an intermediary between the Registry and domain owner. Lexsynergy is one of the world’s most accredited registrars.
Reseller: The company or individual who buys them to sell to their own customers. They have an agreement with a Registrar to sell domains, essentially acting as a "middle man".
In summary: A Registry is the source of domains. Registrars are officially licensed sellers of those domains, and Resellers are companies that have a deal with a Registrar to sell domains.