The rules are few but interesting:
Bikes are 125cc max (so no big BMWs GS accepted!)
Cars are 1.2l max and 8 year old max. again, no big 4X4 allowed, too boring. It has to be interesting!
You ride by teams: for bikes that's 2 bikes.
Ah yes and where to? Get from London to Ulan Bator (in Mongolia) by any route you like. You're on your own! For 10,000 miles!
Mongol rally official website