Trine is a 2D action puzzle-platformer, featuring three characters you can cycle between. There's a Knight with sword and shield, Rogue who attacks with a bow and can swing on wooden areas with grappling hook, and a Wizard who can move objects, conjure boxes, (later on can conjure) bridges and floating platforms, but has no dedicated attack (he can conjure boxes and drop them on enemy's heads though).
The game is very much focussed on physics, and you must use your character's abilities to navigate the areas. I found myself often using the Rogue since the grappling hook could be used in many sections which made things easier. Sometimes you must use the Wizard to place boxes to act as platforms to higher areas, or cover gaps with bridges. The Knight is mainly used for combat scenarios against the enemies.
The enemies usually come in the form of skeletons, but there are times in the game where you are pitted against other enemies such as spiders or ogres. There's a lack of variation overall though compared to other games.
There are plenty of platforms that collapse when you put weight on them, or platforms that tilt on a pivot. These can be balanced out by using boxes or your character's weight. Sometimes you will need to hit certain switches in order to open doors to the next area. There are plenty of traps such as spikes, lava, swinging spiked balls, and falling spiked balls.
The mechanics allow for great co-op gameplay, and produce many moments of brilliance when you work together. You can still switch characters but you can't be the same character, so must communicate effectively to help each other out. The game creates plenty of scenarios that you will find humorous in multiplayer since you could hog a character like the Rogue then watch your friend do things the hard way with the Wizard or Knight.
There's collectibles to find; usually vials that give you experience so you can level up. You can gain new abilities or gain stronger attacks.
If your characters die, you need to reach a checkpoint to revive them. The checkpoints are fairly close together, so this isn't a huge problem. The game is pretty short and can be completed around 4 hours; but it's definitely fun while it lasts.
Trine is a great game, but I'll expect the sequel to have more enemy variety, some new ideas for puzzles and longer length.