What is key id exactly?
User IDs in OpenPGP are used to connect keys to entities like names and e-mail addresses. These are used to search for keys on key servers, and matching them to users/e-mail addresses.
Be aware user IDs are not checked by key servers, make sure to verify them on your own!
OpenPGP key IDs (and fingerprints) are used to reference keys when performing several actions like requesting and sending keys, or when verifying ownership. For example, you'd exchange the fingerprint with the key's owner on a separate, trusted channel to make sure the key really belongs to the person that claims to own the key.
Be aware the eight byte short key IDs do not provide a sufficiently large value space, and it is easily possible to generate duplicate keys through collision attacks. Instead of short key IDs, use at least long key IDs, and when software handles keys, always refer the whole fingerprint.
For more details on how the hash sums are derived, I refer to RFC 4880, OpenPGP, 12.2. Key IDs and Fingerprints which also explains the differences for deprecated OpenPGP v3 keys.
If you want to search for a user ID (or parts of those) first, use gpg --search-keys. This will first query the key servers for the name, and provide some kind of assistant that asks you which keys to fetch afterwards (so, it will automatically run --recv-keys for the selected keys).
The part that looks like CB3AF6E6. GPG also accepts using email address to refer to a key.
How to verify an OpenPGP key's ownership?