Answer:
In many cases, the terms "user ID" and "username" are synonymous. For example, a website may provide a login interface with two fields labeled Username and Password. Another website may label the two fields as User ID and Password, which refer to the same thing. Technically, however, usernames are a subset of user IDs, since a user ID may be an email address, number, or other unique identifier that is not necessarily a name.
(Credits to the rightful owner because this isn't my answer but have a good day :)