Aquaria is a 2d adventure game, reminiscent of games from the 2D era, yet different enough for it to stand out as something special.
You take control of Naija, a mer-creature who narrates her story as you go along. The whole experience can be controlled with your (3 button) mouse, and the introductory area will teach you how to interact with Naija and the world around her. The game is attractive, and the first few minutes are so relaxing, you could comfortably fall asleep, but the game then uses a clever plot device to wake you up and give you a desire to move on and explore.
The game is very open, and reliant on your willingness to explore and figure things out for yourself. As exploration is such a nice experience thanks to the simple and surprisingly effective controls, and wonderfully detailed environments, this openness is not a big problem. Fans of Zelda's last few incarnations may recognize the musical element that activates Naija's abilities, and you can expect a similar feel to puzzles too.
The graphics, with their hand-painted feel, are great, and the audio does an incredible job of creating atmosphere (the plot device mentioned above is a great early example). The mini map could have been better implemented; it's a shame there's no option to leave yourself reminders on the map, as the game is big enough to need it. Aquaria requires time, and may be too relaxed for some players.
The demo is very generous, and will give you a good idea of whether you want to buy the full version.
Aquaria is an atmospheric reaffirmation of 2D gaming that should not be missed