Grails is a web application development framework that uses Apache Groovy (hence, indirectly uses Java) whose main focus is providing a development environment that reduces the number of decisions you have to make when developing without losing flexibility. It does this by combining several things, including opinionated APIs, sensible defaults, and, most notably, a convention-over-configuration approach. 

Grails is an open source web application framework that uses the Apache Groovy programming language. It is intended to be a high-productivity framework by following the 'coding by convention' paradigm, providing a stand-alone development environment and hiding much of the configuration detail from the developer.