Terminology

Label: An individual component of a name, such as 'alice'.
Labelhash: The keccak256 hash of an individual label.
Name: An ENS identifier such as 'alice.eth'. Names may consist of multiple parts, called labels, separated by dots.
Namehash: The algorithm used to process an ENS name and return a cryptographic hash uniquely identifying that name. Namehash takes a name as input and produces a node.
Node: A cryptographic hash uniquely identifying a name.
Owner: The owner of a name is the entity referenced in the ENS registry's owner field. An owner may transfer ownership, set a resolver or TTL, and create or reassign subdomains.
Registrar: A registrar is a contract responsible for allocating subdomains. Registrars can be configured at any level of ENS, and are pointed to by the owner field of the registry.
Registration: A registration is a registrar's record of a user's ownership of a name. This is distinct from the owner field in the Registry; registrations are maintained in the registrar contract and additionally store information on expiry date, rent paid, etc.
Registrant: The owner of a registration. The registrant may transfer the registration, pay rent ('renew' the name), and reclaim ownership of the name in the registry if required.
Registry: The core contract of ENS, the registry maintains a mapping from domain name (at any level - x, y.x, z.y.x etc) to owner, resolver, and time-to-live.
Resolver: A resolver is a contract that maps from name to resource (eg, Ethereum account address, public key, content hash, etc). Resolvers are pointed to by the resolver field of the registry.