(Quick Reference)
                Controller Usage
A controller fulfills the C in the Model View Controller (MVC) pattern and is responsible for handling web requests. In Grails a controller is a class with a name that ends in the convention "Controller" and lives in the 
grails-app/controllers directory. A controller can be created with the 
create-controller command:
grails create-controller org.bookstore.hello
or with your favourite IDE or text editor.
package org.bookstoreclass HelloController {    def world() {
        render "Hello World!"
    }
}Each action in the controller is a method or a Closure and it has access to a number of implicit variables and methods.
Refer to the user guide topic on 
Controllers for more information.