What to use: Class or Module, in ROR

I am new to ROR and trying to convert my PhP application to ROR application. There is one php file common.php which is included in almost all other files. Likewise common.php in turn uses RPCClient.php and User.php . Variables and methods are shared across php files. So, what should I use in ROR a class or a module as data is shared among various files.

Any kind of help is much appreciated, Warm Regards.