Grails is an open-source web application framework that simplifies the development of web applications using the Java platform. It is built on top of the Spring framework and leverages the Groovy programming language, which is known for its concise syntax and dynamic capabilities. Grails follows the convention over configuration principle, allowing developers to focus on writing code rather than dealing with complex configurations. One of the key features of Grails is its ability to integrate easily with existing Java libraries and frameworks, making it versatile for various projects. It also includes built-in support for RESTful APIs, database interactions, and testing, which streamlines the development process and enhances productivity.