Pokémon Mystery Dungeon: Explorers of Sky is essentially a slightly expanded merged title of the two previous Mystery Dungeon games, Explorers of Time and Explorers of Darkness (much like the main Pokémon games received expanded compilation games some time after release).
Unlike the regular Pokémon games (Pokémon Diamond, Leaf Green, etc), the Mystery Dungeon takes humans almost entirely out of the equation and instead features Pokémon adventuring by themselves. The story suggests that this version of reality doesn't feature humans at all, with Pokémon performing all sorts of (usually human) functions such as shop keepers and so on.
The player is tasked with taking control of their Pokémon (selected via a form of personality test), from 16 of the 493 known Pokémon. They're also able to choose a partner who will befriend them and help them through the game.
Players directly control the Pokémon as they attempt to complete jobs that take place in randomly generated dungeons, battling any "bad" Pokémon that they might encounter.
Control is a pretty straight forward affair, with the D-pad controlling movement (there are touch screen options but traditional button-based controls seem to work best), A performing your basic attack and L+A controlling your assigned special move. Combat is turn-based, all other Pokémon taking a turn when you move or attack. You can link up special attacks at a Pokémon dealer back in town, once you've advanced far enough, but you can also jump in to the pause menu to mix things up if you run out of power for your various moves.
As a reboot of the franchise, the Mystery Dungeon series hasn't gone nearly far enough to bring the gameplay up to date. Combat is frustratingly light on tactics or skill while managing the various items you can find, and your party's inventories is an arcane replica of the decade-old Game Boy interface. It seems the designers are unable or unwilling to mess with the ancient formula and, as a result, it will only be as good as to be "cumbersome" after spending some time figuring out how it all fits together. Usability is not a word ever used during the development of this title.
Death (or fainting, as it's called here) is terminal to your progression within a dungeon. Should you be defeated (and, given the random difficulty of one dungeon to the next, it will certainly happen), you will be ejected from the dungeon, losing half of your items (at random) and ALL of your money. This old-school rogue-like gameplay mechanic predates even the original Pokémon game and feels bizarrely harsh for a game that's otherwise clearly intended for a younger Pokémon aficionado. If you're fortunate enough to have friends with the game and access to the internet, you can send out an SOS and have them come rescue you, but that's a niche within a niche and is unlikely to be something you can rely on much of the time.
Technically there's not much to say - it's a DS game and it knows it, never daring to stray into the realms of technical complexity and using its two screens well. The various Pokémon are well realised (as you'd expect, the assets for the various Pokémon should have been readily available from the other DS games) and the sound presentation is up to the graphics. It's nothing flash but it never drops the ball either.
Explorers of Sky is a bizarre mix of child-like presentation with hardcore gameplay mechanics which have long since been abandoned by most other hardcore games. It's simultaneously simple and obtuse, easy and challenging. Somehow it manages to juggle incompatible ideas while still managing to be charming and addictive - just be aware that the addiction comes at a price; you'll be frustrated that you want to keep playing it.