This DLC pack contains a new co-op multiplayer mode known as Siege, two new multiplayer maps, six new character skins and 11 PSN Trophies (10 of which are bronze, one of which is silver). Two of the six new skins are from Uncharted: Drake's Fortune and the other four are new and exclusive to the game.
Source: Wikipedia, "Uncharted 2: Among Thieves", available under the CC-BY-SA License.
"Siege" cooperative game type:
An arena co-op game type where two to three players defend a marked territory within a map against waves of NPC opponents. At the end of every round, the territory will change locations within the map and the NPC opponents will increase in difficultly. The only way you can score co-op points is to have all your players within the captured territory, all while being assaulted by NPC opponents.
1. The Highrise:
Towering over the streets of Nepal, The Highrise is a map with plenty of opportunities for players to get the high ground on their opponents. Set on a series of destroyed and under-construction buildings, players need to be on their toes as there are many different jump locations and paths around the map. Players should also watch out for the incoming airstrikes, as they can clear the rooftops in seconds.
2. The Museum:
Set at dusk amongst the courtyards and display cases at the site of Drake and Flynn’s heist, The Museum provides players with a mix of intense gameplay covering traversing across rooftops, dodging gunfire through twisting corridors, and close quarters fighting. Players need to watch their backs as there are numerous flank routes in all parts of this circular map. Players should also learn the secret climbing routes that will allow them to get the drop on their opponents.
Source: The Developer Website.