The first two parts, the overview and the instanced nature of skirmishes, go over what the new Skirmish system provides players, and how skirmishes put a twist on the old concept of instancing. Did you ever want to defend the Prancing Pony in Bree? How about fight along side Elrond's sons? Skirmishes will be opening up those options to players while still offering repeatable adventures for any group size.
But we shouldn't talk our faces off here! Go on over and read Brian "Zombie Columbus" Aloisio's developer diary on the whole matter. He can describe it better than we can!