What each one is
ORCID is a persistent identifier you own. You register it yourself, it is free, it follows you between institutions and countries, and you decide what appears on it. Crucially it is publisher-neutral — no database company controls it.
A Scopus Author ID is assigned automatically by Elsevier. You do not create it; Scopus generates it by clustering documents it believes belong to one person. You can request corrections, but the identifier exists whether you engage with it or not.
ResearcherID, now folded into Web of Science author records under Clarivate, plays the equivalent role in that ecosystem. Same idea, different database, and the same automatic-assignment problem.
ORCID is the one you own and control. The database identifiers are assigned to you, whether or not you ever look at them.
Why you need more than one
They are not competing standards to choose between. ORCID is your portable identity — the one you put on a submission form, a grant application or a CV. The database identifiers are how two specific commercial databases track your output internally.
Because the database identifiers are generated automatically, they are where errors appear. A name spelled two ways, a change of institution, or a common surname can produce several identifiers for one person, or one identifier holding two people's papers.
The practical arrangement is to treat ORCID as the authoritative record and connect the others to it. Both Scopus and Web of Science can push confirmed publications into an ORCID record, which keeps it current without manual entry.
Setting it up properly, once
Register an ORCID iD if you do not have one, and add it to every submission from that point on. Most journal submission systems now ask for it, and some require it for corresponding authors.
Then find your Scopus Author ID and your Web of Science record and check what each contains. Look for papers missing, papers that are not yours, and duplicate profiles under name variants. Fixing these is a separate job covered in the profile-merging page in this cluster.
Finally, connect them: authorise the Scopus-to-ORCID and Web of Science-to-ORCID links so new publications propagate automatically. This takes about twenty minutes once, and it stops the record drifting apart again.