It is free to sign up, you can either create your own event or join one someone else has created.
There is always a long list of stuff to join and so far my experiences have been positive.
They use your battle tag (Not RealID) to do cross realm events.
Here is the link, or you can just google openraid.
This is a great way for people who are looking to finish off a legendary or get your last few achievements completed without having to count on your local servers population.
And no, this isn't malware, and no I do not work for them. I just think it's such a great tool I wanted to get this page out there for players who may not know about it.