TL; DR: Underrail is possibly the closest thing we'll get to an actual Fallout 3, and it's way less buggy than a Ubisoft/Bethesda game two months after release.
Underrail is an isometric turn-based post-apocalyptic game, inspired by Fallout and other cRPGs. It features a classless system just like Fallout: there are no classes, and so your character is defined by his skills and attributes. In terms of skills, you have weapon skills (guns, melee, grenades and crossbows - crossbows are unique because they have a great variety of special ammo), "rogue" skills (hacking, stealth, lockpicking, traps and pickpocketing), crafting skills (mechanics, science, biology, chemistry and tailoring - we'll get to crafting later), social skills (persuasion, intimidation and bartering), defense skills (dodge for melee, evasion for ranged) and psi ("magic") skills (thought control, metathermics - ice and fire - and psychokinesis.) You also have feats, which are both passive bonuses and active abilities which are unlocked throughout the game. Basically, this thing has some ♥♥♥♥♥♥♥ replay value.
Combat is like Fallout, but with several additions: psi "magic", special ammo, great variety of grenades. Avoiding combat is also an option: several levels feature vent shafts and you can usually avoid enemies if you invest enough in the Stealth skill. Also, you can alert cameras, which divert, for example, Security Bots to the camera position in order to get past another point.
The setting is very interesting, somewhat Metro-like: it's impossible to live on the Earth's surface, and mankind lives underground, connected by an underground train line called the Underrail. There are several factions throughout, and you might get to choose which faction gtets to rule a city, for example.
In terms of graphics, the game is nothing shiny, but it gets the tone: dark blue caves, grey bunker walls with green lights, etc. It looks pretty good in that regard (fun fact: the textures of backgrounds were made in the XP version of MS Paint, and then with some added effects on GIMP!)
So, I'd recommend you buy this game, especially for the price (come on, 8€/10$ is a steal for this!) and have fun! (another fun fact: the game was made by one guy alone, with the exception of soundtrack and if I'm not mistaken, character sprites and the main menu art for the game!