A registered participant is one who has completed the site sign up form, so all required registration details of the person are known. To view all your participants, click on the Participants link in the navigation bar of EHQ. All your participants will be visible in the Participants page, as indicated below.
A project subscriber is one who has clicked on the follow project widget and only provided his/her email id. No other details of this person may be known. Read the article How many people are following my project to get the details of your subscribers.