A subname is a name created one level below a parent ENS name — e.g. shop.yourname.eth is a subname of yourname.eth. It has its own records, its own resolver, and (optionally) its own owner, but it exists inside the namespace of the parent.

Conceptually yes — the term "subdomain" comes from DNS and many people use the words interchangeably. In ENS terminology, "subname" is the preferred term. The mechanism is similar to DNS subdomains in that the parent delegates a portion of its namespace to a sub-entity, but the storage and ownership model is on-chain rather than via zone files.

Creating a subname is a two-step job in ENS Explorer: deploy a subregistry on the parent name, then create the subname under it. You need to own the parent name. For the full steps, see Create a subname.

Creating a subname has only the gas cost of the transaction — there is no registration fee the way .eth names have, because the parent's owner is the authority. If the parent name uses a subregistry (a contract that manages subname allocation), that subregistry may charge its own fee — but that is a decision of the subregistry, not ENS itself. Many name communities hand out subnames for free; some sell them.

By default, the creator of the subname owns it. Subnames can then be transferred to other addresses via role grants (Explorer has no single "Transfer" button — the Can transfer admin role on the Add-user form is the primitive). On ENSv2 subnames managed through a subregistry, each subname is an ERC-1155 token and can be transferred like any other NFT. For legacy ENSv1 wrapped parent names, subnames are wrapped NFTs in the Namewrapper contract.

Yes. A subname can be set as a primary name just like a .eth second-level name — so alice.yourname.eth can be what apps show for Alice's wallet. See What is a primary name?