Simply put, it's a rogue-like squad-based strategy game, but it is much more than that. This amazing indie game features elements of tower defense, RPG, 4X Strategy, dungeon crawling, and brings it all together in a unique experience.
It is similar to FTL in that it is addicting and you can replay the game multiple times with drastically different experiences (different team heroes, sometimes you find the exit right away, others you comb through the entire dungeon), and you progress by unlocking new (characters and) ships, which unlock new modes.
The normal difficulty is actually quite hard already, and if you unlock the Sanitary ship it is the real Hard Mode (which is basically impossible for new players lol). Try your hand at it. You will be pleasantly surprised :-)
EDIT: I had to add this in, because it is noticeable enough to be mentioned. This game has a few bugs, especially the multiplayer side of it. Although the developers patch the gamebreaking ones quickly, there are some minor ones that are annoying. Amplitude Studios makes buggy games, but they're good at bug-fixing. Take that as you will lol
Last point, I would not buy this for the co-op multiplayer mode, unless you have friends to play with. Public matches are filled with leavers, afkers, and since the game lasts 4-5 hrs to complete, its not reasonable for 4 strangers to stay that long for 1 run.