As for the gameplay, everything is instanced. You start every kind of mission from Dome City, a non-combat area that contains all the various shops as well as the mission board. To enter a mission, you use the board, or hit hotkey "M,"and queue yourself and/or your team for chosen mission. Each mission has a set number of participants, so you'll be grouped with random players also in queue unless you have a full group. The types of missions in Global Agenda are split into two types of gameplay: PVE and PVP. Since the PVP play is such are large draw for the game, I'll spend a few words on it before I go in-depth to the PVE gameplay (which is the co-op part of the game). There are three different PVP modes: Mercenary, 4v4 Arena, and Conquest. Mercenary mode is 8v8 or 10v10 team-based PVP, divided into five different types of gameplay over several different maps, most types being very reminiscent of types seen in Team Fortress 2 (e.g. Payload is pretty much exactly the same, conceptually). 4v4 Arena simply requires you queue with a full 4-player team and duke it out over a control point to gain the most points. Conquest is the large scale world PVP, offered to monthly subscribers. You must be part of a reasonably large agency and/or alliance to participate in Conquest. You receive credits and XP for all PVP modes.
One pleasant surprise I should note: when I first started playing, there was no level sorting to PVP games, so one would often end up in a Mercenary game with level 50 players against level 5 players, which felt a little unfair at times. In the most recent patch, Hi-Rez implemented a system where in Mercenary mode, players will be matched with other players in their general level range. This definitely made things feel a lot more balanced. Kudos to Hi-Rez for putting this in finally.
So what does the PVE portion of the game have to offer? There are 5 different PVE modes in Global Agenda, which are mostly divided by level: Low Security, Medium Security, High Security, Maximum Security, and Double Agent. These different modes are unlocked with levels (e.g. High Security requires level 12), and after a certain level, certain modes will stop giving you rewards (e.g. Low Security stops giving you rewards after level 9). Low, Medium, High, and Maximum Security modes are pretty much cut of the same cloth, except each sequential one is higher in level requirement, takes longer to do, and offers better rewards. These modes are 4-player co-op modes with the goal being to make your way through the level to take out the boss. After completing the mission, you'll receive XP and (possibly) credits, depending on how well your team preformed. Additionally, there's a "challenge" condition: if you make it all the way through the mission with less than four total deaths on your team, you get extra credits and XP.
Global Agenda is different from other MMOs in the fact that you don't get XP or money until at the end of a mission; killing enemies won't give you XP, for example. Mob kills will, however, net you crafting materials, which are automatically distributed to party members. This is a huge advantage to PVE; while I can't speak for Conquest mode, as I haven't participated in it yet, the other two PVP modes definitely did not net me any crafting materials. I've also heard that the higher level PVE missions drop high level blueprints for upgrade items (you can only buy the lowest rarity of upgrade blueprints from vendors). While PVP appears to be a faster way to get credits in Global Agenda, PVE is definitely the way to go for crafting materials. Double Agent mode has the standard 4-player team, but there are also two other players that will be chosen to fight for the PVE faction the Commonwealth. So this is kind of PVE meets PVP.