Robot models in simulators

I am considering building models of my robot in simulators to check things out.
I came across V-REP as a possible simulator.
In what format are robot models built? Are any standard models available? How can you build a model for your robot?
To start with, I want to have models for SainSmart 6 dof arm and GoPiGo available in a simulator.

In what format are robot models built?

That would depend on the simulator.

I use CATIA and DELMIA to build robot models and program the robots to do useful things. The robot parts are individual parts in a product, and the joints are fixed joints (this part holds still/is bolted to the table/floor) or revolute joints (this part rotates about this line that is colinear with that line in that part, while these faces remain in contact (or a fixed distance apart)).

Of course, these two products are outside the price range of most hobby robot fans.

Are any standard models available?

All the major robot manufacturers supply STEP files that define the parts of their robots. You can then import those STEP files into your preferred CAD system, and easily define the joints.

Thanks @PaulS. I came across V-REP simulator a few days ago and it is free.

I got my simulation environment up and running (Robot simulator with a python interface - YouTube). It is an environment where I can test things before implementing on real hardware.