Customers have a special web interface in OTRS. Through this web interface customers can create new accounts, get an overview on own tickets, create and edit tickets, change the account settings, e.g.
Lets say the host with the OTRS installation is reachable via the domain http://www.example.com then the login screen for customers can be reached by the URL http://www.example.com/otrs/customer.pl with a web browser.