Self-enrolment: the user sees a "Enrol me" button and click it to enrol in the course. You can set limitations about who see this button and choose wich role to assign.
Auto enrol plugin: the user has not to click any button, he is already enrolled in the course as soon as (your choice) he logs in Moodle or accesses the course the first time.
Auto enrol plugin: the user has not to click any button, he is already enrolled in the course as soon as (your choice) he logs in Moodle or accesses the course the first time.