The registry is a single entity that decides which names exist under a particular domain hierarchy, providing this capability might be contracted out to somebody else to do on behalf of the "real" owners of that domain for example the COM domain registry is operated by Verisign under contract.
One or more Registrars provide (mostly sell) the service of managing entries in that registry. For all the popular commercial TLDs there are multiple commercial registrars. The registrars have to abide by rules set by the registry (since if they did not the registry can just stop them providing any services) even if they are not under the same legal jurisdiction as the registry.
For the gTLDs the rules for this playing field are set by ICANN. It would like these rules to apply everywhere, but the ccTLDs are assigned to sovereign entities, and like a two year old, sovereign entities do not take kindly to being told what to do.
