Eversion is difficult to review without spoiling it, but I'll try.
Eversion is, at it's core, an indie platformer/puzzle game. You play as Zee-Tee the Astrik (*), who is on a quest to save Princess Nehema, who has been captured by things that look a lot like Goombas. Along the way, Zee-Tee can evert to different "layers" of the world by pressing SPACE in certain areas, Things are quite different in each eversion layer, so everting is often required to progress.
Along the way, Zee-Tee can challenge himself by trying to collect all 240 gems, which will often require backtracking at certain eversion layers. Thankfully, your gems save even if you quit the level or die within, which is great for some of the harder levels.
The only real complaint I have about this game is how short it is. The game can easily be 100%ed in an hour, but if it is your first time playing, it'll probably take 2 hours or so to collect all of the gems and rescue the Princess.
All in all, Eversion is a very interesting platformer that is worth taking a look at. If you want to try the game out before you spend 5 dollars on it, the creator has a freeware version on his website. It doesn't have achievements and it's missing a handful of time attack levels, but it should get you used to the game.