From the website:
You just joined Robin Hood's merry men in Sherwood forest. You can help at the Sherwood forest camp, rob travellers on the road to Nottingham castle, bring messages from Robin, and scout the neighbourhood for information.
The more risk you take the higher the rewards, but be careful : If you get too close to the castle you will need to go back and hide in the forest before the sheriff's guards can catch you!
You want to be the first to accomplish 7 missions of the same type in order to become Robin's new lieutenant. Of course your friends want to become lieutenants too so you will have to be swift and daring to succeed before them.