I am planning to deploy an application on AWS. It is an application that exposes some REST/Json webservices.
It has a CMS UI created used active_admin.
Can I deploy that system in AWS? Or it is intended to be used only to deploy webservices systems?
I think what you are looking for is Amazon EC2 (which is part of AWS). You can setup a small instance pretty quickly and that should let you deploy your rails application.
In addition to Joe's answer, EC2 gives you a virtual server. You can log on the the virtual computer with ssh and install any software you need.
You don't even have to run a web server - if you wanted to run a program to process information (ie, render computer graphics images), you could do that as well.