It's pretty much just going over land in a vehicle. I guess you live in the vehicle too. I'm not totally sure of the requirements. But there are some pretty amazing journeys being taken by some really adventuresome people (who also must be incredible packers).
See some of the cooler ones I've found below:
Overlandia (okay, it's just pictures)
A Restless Transplant
How did I get wrapped up in this? From following links on A Continuous Lean's site. I've been checking out that site for a long time now, and it's a pinnacle for American made, craftsman, waxed-canvas, outdoorsiness, reusable, slow-movement kind of stuff. I love the the type of style, but I just can't seem to calm myself down enough to be that simple!