Visitors are anonymously tracked using session ids throughout your site. But when visitors identify themselves at login, signup or checkout, you may pair a session id with a user to identify the entire chain of events one user has carried out over time.
/* Identify a user */zlt.track('user', {'user_id':'john.doe@userdomain.com','email':'john.doe@userdomain.com','first_name':'John','last_name':'Doe'});
The user_id should be a unique identifier. It can be an email address or any other identifier of choice.