Standard PvE missions typically involve working your way through the mission complex and defeating a final boss at the end within a set time limit. During the mission you'll typically encounter a large number of small, weaker mobs, a varying number of harder mobs (depending on the difficulty), and a much tougher boss mob. Killing the boss mob will complete the mission (though you'll likely need to kill the other mobs to accomplish that). Many boss mobs have some sort of special abilities, whether it be a personal shield, a powerful vortex that sucks players in, projectiles that can stun players, or some other such mechanic.
Standard PvE is done with teams of 4 players. You can either form a premade team and then have the entire team queue for a mission, or you can queue with fewer than 4 players and the matchmaker will locate other players to join your group.
Each mission has a bonus meter - each time you or someone on your team dies, this meter fills up; if it fills completely then you won't get bonus XP for the mission (but you'll still get regular XP). Thus, it's a good idea to try to avoid dying if possible, since more XP rewards are always nice.
There are currently 4 difficulty levels for standard PvE: Low, Medium, High, and Maximum Security.
Low Security missions are designed for players of level 10 or below (and will not grant XP to characters above level 10). These missions typically have only a single Elite-class mob serving as the final boss of the mission.
Medium Security missions are designed for players above level 10. These missions tend to have individual Elite-class mobs every so often throughout the mission, plus a true boss mob at the end.
High Security missions are designed for experienced players (and require a minimum level of 12). These missions will often have multiple Elite-class mobs fighting the players simultaneously, and a more difficult boss at the end.
Maximum Security missions are designed only for truly coordinated teams (and require a minimum level of 22). Large numbers of Elite-class mobs and very difficult final bosses will challenge most teams.
PvE missions grant XP based on a number of factors: successful completion, number of enemies killed, and whether or not the bonus condition was met. PvE missions also provide random item drops (upgrade crafting components and blueprints). While PvE missions do not give credits as a reward, the item drops if undesired can be sold to vendors or other players in exchange for credits.
While there are a set number of overall maps for PvE missions (dependent on the security level of the mission queue you select), each of these maps is designed in such a way that specific elements of the layout and the mobs contained within the map may vary. Thus, don't expect particular mobs to be in the same spots they were in previous runs, nor that the exact same cover might be available when entering a room.
Once your team reaches a certain point through the mission map, a checkpoint beacon will activate. Once the checkpoint is activated, you'll be able to use the beacon entrance within the dropship to teleport back to the checkpoint if you die. (To use the beacon, just stand in the glowing rings that should have sprung up from the previously-inactive entrance on the floor.)
The entrances to boss rooms are typically guarded with strong forcefields (glowing orange walls). Only one of these forcefields will allow you to enter, and once you have entered a boss room you will be unable to leave (except through dying). It's generally a good idea to make sure all of your team is present and ready to engage the boss before stepping through the forcefield.
Lost Your Way?
If you get lost or turned around within a PvE mission and aren't sure which way to go, press your Pathfinding key (by default, N) and a trail of glowing arrows will point you in the proper direction. (Note - this won't always work if you're inside the dropship; exit the dropship and move forward a bit and try again.)
Asmo - Knowledge is Power