Plesk is a hosting automation solution that simplifies setup and management of user accounts, websites, and email for hosting providers. It manages common software components like web servers, FTP servers, DNS servers, databases, and more. Plesk's user account hierarchy includes client accounts for resellers, domain administrator accounts, and email user accounts. Client accounts can host multiple sites and resell hosting, while domain administrators are limited to one site. Plesk comes preconfigured to immediately start serving customers after installation.