WooCommerce is a WordPress plugin that adds eCommerce functionality to a WordPress site, allowing users to design their site through the WordPress designer and utilize tens of thousands of third-party plugins. Before installing WooCommerce, users need to register a domain name, find WordPress hosting, and install WordPress. Installing WooCommerce involves searching for the plugin in the WordPress library, clicking Install Now, and then activating it. Once activated, users can access WooCommerce's features and design their site by going to Appearance or Products.