cPanel is a fully feature web-based control panel that allows you to manage your domain through a web interface. You have the ability to manage all aspects of email, files, backup, FTP, CGI scripts, web site statistics, install software (see Fantastico below), and more.
Fantastico is an add-in for cPanel. It allows you to easily install applications that are (mostly) free. Fantastico greatly simplifies installing these popular packages by removing the hassle of researching software, downloading it to your computer, uploading it to the web server and installing it.
WebHost Manager is the control center of the cPanel/WebHost Manager Package. It is used to setup and manage accounts, monitor bandwidth, disk space usage and services, and much more.