Hi Sanjay,
The article that Raghavan linked to is a good one in discussing some differences, however it is a bit old and many of the issues Matt Raible brings up have been addressed.
Grails may not be quite as "mature" as Rails, but the frameworks underlying it (Spring, Hibernate,
Ant, etc.) are much more mature than Rails.
Grails is a
JEE compliant web framework so it adheres to the Servlet spec, but you don't have to deal with Servlets at all to use Grails. An understanding of how they work can be helpful at times but is rarely needed.
Groovy is a dynamically typed (or optionally typed) object oriented language for the JVM and Scala is a statically typed functional / object hybrid language for the JVM. I don't know much about Scala other than that so I won't try to compare the languages further, but Groovy is gaining adoption at a rapid pace and is a joy to work with!
If you check on sites like indeed.com you will see that Groovy and Grails, though still a minority of the job market, are growing quickly. I have worked on Grails full time for the past two years (at three different organizations).
I hope that helps,
Dave