Dungeon Defenders is basically a first person tower defense game with an RPG element in it. You create a character out of the four main classes: apprentice, squire, ranger and monk. Each class has it's own abilities and towers which you must use to defend your eternia crystals (which are your cores) against wave after wave of vile mobs. By defeating these monsters your character gets awarded experience and loot, which you both use to upgrade either your character and/or your towers, thus making it possible to reach even higher waves of massacre.
The game features a (simplistic) story about the four main classes. The story progresses trough the several campaign missions. These mainly serve as a tutorial to the game and provide as a means of learning the basic gameplay components and teamwork aspects. Later on these same missions can be done on insane or nightmare difficulty to provide a challenge even for those who already know the tactics.
As noted above, the game can be played in multiplayer mode, but this does not prevent you from playing solo. Most multiplayer maps can carry up to four people, however there are some exceptions of 6-player maps and even bigger multiplayer PvP matches. The advantage of playing multiplayer games over solo games is that it's a lot easier to maintain your towers during waves. Your towers get damaged by monsters and need upgrading and repairing in order for you and your team to survive. From my own experience I can guarantee it that you can get overrun pretty fast, and having an extra teammate to back you up here can often be the only way to survive the game!
Besides the normal multiplayer maps, there is also a wide variety of challenge maps, which can be found under the "challenges" section of the game. These offer situations not normally encountered in the game and require special strategies in order to beat them on the harder levels. The rewards received from completing these are often unique and rare items.
What I like the most about this game is the fact that you can just fire it up at anytime, and have a good time just joining up with random people. The character development system is also one of my favorites as it provides a lot of possibilities because of the random looting system, though, a lot of grinding is involved to reach some of the better gear.
If you're into these kinds of games, pick it up on steam or amazon, it's worth the price.