In Guild Wars 2, cooperation generally means players working together in a number of multiple, intuitive ways. Key mechanics related to cooperation are dynamic level adjustment and dynamic events, as well as encouraging of players to help each other by reviving each other and helping defeat enemies for shared, equal rewards.
In Player versus Environment, sometimes referred to as "cooperative" game mode, players fight against common foes or work to achieve similiar goals. Because all professions are effective in dealing damage, controlling enemies and supporting allies, all players can play a useful role in combat situation and during other tasks.
Dynamic level adjustment and dynamic events in conjunction with each other ensure that players participating in events or other gameplay can not completely overshadow lower level or less well-equipped players by destroying enemies or completing other objectives before others can participate. This is especially true for group events, where all participants need to contribute in some way to the fight; an example would be a higher level character dealing with a boss-like foe in close quarters combat while other characters help deal damage and revive each other should they become downed.
Another example of cooperation in PvE is the distribution of loot and other rewards. All players contributing for a kill or event objective will be rewarded equally, according to their own level. As long as players contribute a certain amount of damage which depends on overall strength and health of the enemy, they will receive same rewards as everyone else who helped to defeat the enemy.
Dungeons don't benefit from player cooperation; they require it. All dungeons (excluding some Living Story solo dungeons) are adjusted for no less than five players who are required to be in the same party. Enemies in dungeons are much stronger than normal opponents, and especially boss creatures in dungeons require additional coordination and team effort.
Personal storyline, while largely soloable content, can also be played by more than on player, as long as they are in the same party. All players in the instance will have their level adjusted to the level of the instance.
While traditional trading cannot be practiced between players personally, all players have access to Black Lion Trading Company and the Trading Post market. While not strictly speaking emphasizing cooperation, trading post system allows players of all worlds to indirectly trade each other through a controlled interface, greatly reducing harmful activities such as scamming or selling of gold for real money from being practiced by malicious players. A global trading system also ensures that all worlds take part in same global game economy without any of them sinking to a virtual regression which would cause players to abandon the world.
Finally, many major elements of the game such as crafting, gathering or map completion can all be practiced together with other players. Many other systems put in place also facilitate cooperative play; players can use contacts interface to manage individual friends and contact them as well as banding together to form guilds, which play a major role in all aspects of the game.
World versus World is part PvP, part PvE. Players are required to work together to defend, patrol and siege objectives held by two different opposing forces consisting of players of another worlds. Many tasks in WvW are simply impossible, extremely difficult or more time-consuming to accomplish alone; for example, deploying of siege weapons or repairing of damaged wall sections or gates require more supply than a single player can carry at a time.
While players can band together into a massive group known as zerg to assault enemies towers or keeps, a more organized group led by a commander can help guide these players in taking key objectives instead of randomly assaulting anything in vicinity. Guilds can claim WvW locations for their own, providing bonuses earned with influence to any ally nearby, and even small groups of just a few players can take over less well-defended locations such as resource camps and sentry points.