User agreement is the name a service gives to its contract with the individual using it, and it covers the same ground as terms of service or terms and conditions. The phrase is most common where there are accounts, because the document is about an ongoing relationship rather than a single purchase, and that changes which clauses carry the weight.
What makes it an agreement rather than a notice
Two things. The user is offered it before they get the service, and they do something that counts as saying yes. A tick box, a button labeled with what pressing it means, or a sign-up flow that presents the document at the point of registration. What you keep afterwards is the version in force, the wording shown and the timestamp, and those three together are the difference between an agreement and a page on your site.
The clauses an account relationship needs
Registration and account security, so responsibility for what happens under an account sits somewhere. The plan and the billing, including renewal and the route to cancel, described in the same words the product uses. Acceptable use, as a specific list. Suspension and termination on both sides, with what happens to the user's content and data on the way out. A user content license, if users upload. And the change clause, because a relationship that runs for years will need the terms updated and doing that without a mechanism means nobody is on the current version.
Changing it without breaking it
Publish a new version with a new effective date, keep the old one, and give the notice your own change clause promises. For material changes, ask for acceptance again at next sign-in rather than relying on a banner, because the record of a second acceptance is worth more than an assertion that notice was given. Quietly replacing the document leaves every existing user on the version they agreed to and you without a reliable way to say which that was.
Questions people ask about user agreement
Is a user agreement different from terms of service?
No. Different name, same instrument. The phrase is just more common where accounts and an ongoing relationship are involved.
Does it have to be signed?
Not in the handwritten sense. Electronic records and signatures have been recognized in commerce by federal law since 2000; what matters is evidence that this user agreed to this version.
Can it cover data protection too?
It should point at the privacy notice rather than restating it. Consent for processing, where a regime requires consent, has to be a separate specific act rather than a term of a contract.