Yes, each new visitor session (has no cookie) is treated as a Guest Customer. nopCommerce uses an Access Control List to control access to most aspects of the website. For example, some functions can be configured to work for visitors who are not yet customers at all(Guests)..such as posting reviews, or comments on blog articles or news articles. Guests are also (by default) allowed to shop..including put items in their shopping cart. The Guest Customer is converted to a Registered Customer during registration (optional), which can occur prior to, or during checkout. Or an administrator can set the appropriate "Role(s)" for the customer from within the admin area.
Other customer roles include: Administrators: can fully administer the website Moderators: can manage all aspects of the Forums Vendors: can manage various aspects of the adminisrative system
Every customer can be in as many Roles as is appropriate. For example, you might have a customer who makes purchases that is also working to moderate your forums..they would be "Registered" AND a "Moderator"