Grails is a web framework for Groovy that follows the example of Ruby on Rails to be an opinionated web framework with a command-line tool that gets things done really fast. Grails uses convention over configuration to reduce configuration overhead.
Grails lives firmly in the Java ecosystem and is built on top of technologies like Spring and Hibernate. Grails also includes an Object-Relational-Mapping (ORM) framework called GORM and has a large collection of plugins.
Different versions of Grails can be very different, so you need to take care when upgrading your Grails application, especially with major versions (2.4 to 3.0, ...
