In layman's terms, what is the "open source" movement and what does it aim to change about today's society?

So what is the "open source movement"? The little snippets of information that I’ve managed to gather tell me that the proponents of the open source movement want the right to be able to modify and redistribute software as they wish. So does this mean that they’re unsupportive of the current software copyright laws?

Is there a difference between open-source and free software?

I apologize for my ignorance, but I’ve only recently discovered an interest in programming.