What is a login API?
The Login Service API provides endpoints to manage the User Session in Conversational Cloud, such as User Login, Application Login, Logout and Refresh. Use this API to log into Conversational Cloud as a user with credentials or an application with an API key.
How do I log into REST API?
With some authentication schemes, there's no traditional login. REST does not dictate any security rules, but the most common implementation in practice is OAuth with 3-way authentication (as you've mentioned in your question). There is no log-in per se, at least not with each API request.
What is an API user?
The user API allows you to create, read, update and delete users from your account. It also has specialized methods to log in a browser while creating or updating a user object at the same time. The API accepts data as URL parameters, as JSON or as XML, and can return data as either JSON or XML. ...
How would you log in to some site using API if you have the credentials?
The most simple way to deal with authentication is to use HTTP basic authentication. We use a special HTTP header where we add 'username:password' encoded in base64. Note that even though your credentials are encoded, they are not encrypted!