mapping

I am a bit confused about how climbers map routes up snowy mountains. An article on MountEverest.net shouts about how a new route has been found on Annapurna in the Himalayas (the picture on the right shows the classic route as a red dotted line and the new route in green. I think the photo on the left just shows old ropes some careless climbers left behind). Surely there must be a million different routes you could take, just by taking a side step. I wonder what the rules are.