WooCommerce is a free WordPress plugin that allows users to easily build an online store. It provides the basic functionality for an online store out of the box, including the ability to sell physical, digital, and service-based products. To build a WooCommerce store, users need to obtain a domain name and web hosting, install WordPress, add the WooCommerce plugin, create essential store pages like Shop, Cart and Checkout, set up payments and locales, add products, customize the design, and extend features using additional plugins. WooCommerce stores can be customized in many ways to create an effective online storefront.