It's pretty much like playing Phantasy Star Online. You meet people in a shared lobby, then you start a party and go with up to 4 people into an instanced dungeon. The dungeons are procedurally generated out of pre-made pieces and mobs spawn there. You beat the shit out of them and take their stuff. Then you run through it until you reach the boss, beat the shit out of him, and take his stuff. Repeat until you're level capped or don't want to anymore.
The combat is based on comboing your basic attack into smash attacks, you can pick up almost anything and use it to bash the mobs, there's a dodge skill like in Tera, and the bosses are pretty fun in terms of figuring out their patterns and learning to avoid them. Or, you can just pick up bits of the environment and chuck them at the boss to stagger them and then whale on them until they get up.