The Create User (username) email notifies users of their account creation and username. The Create User (password) email notifies users of their system-generated password.
Make sure that both of these templates are activated under Communications > Email Templates so that your users will receive their log in credentials. These emails will also go out when you import Users in bulk using the Excel template.
To customize these templates, go to Communications > Email Templates, and click on the edit icon for the Create User templates.