CouchDB stores the information about the database users in a special database, the authentication_db. The default authentication_db is called "_users". So this is where your user data is saved when you sign up without taking care about the details, e.g. when you use the signup method provided by the JQuery HTTP API. Read more about the ways to interact with the authentication_db in the HTTP API documentation (JQuery methods), and read more about general CouchDB security in the CouchDB Wiki or in The Definitive Guide.
How to set the authentication_db
When you e.g. create database users from within your tests, how can you make sure they don't mess up the actual authentication_db, the one the unsuspecting user might use in production?
What is this HTTP API?