(Quick Reference)
                params
Purpose
A mutable multi-dimensional map (hash) of request (CGI) parameters.
Examples
To obtain a request parameter called 
id:
class BookController {
    def show() {
        def book = Book.get(params.id)
    }
}To perform data binding (see 
Data Binding in the user guide):
def save() {
    def book = new Book(params) // bind request parameters onto properties of book
}Description
The standard Servlet API provides access to parameters with the 
HttpServletRequest object. Although Grails provides the same capability through the 
request object, it goes a bit further by providing a mutable map of request parameters called 
params.
The 
params object can be indexed into using the array index operator or de-reference operator, so given so given the URL 
/hello?foo=bar you can access 
foo with
println params.foo
The params object can also be used to bind request parameters onto the properties of a domain class using either the constructor or the 
properties property:
def book = new Book(params)
book = Book.get(1)
book.properties = params
For further reading see 
Data Binding in the user guide.