Gameplay
Undertale's gameplay is fairly simple, but unique. It's a top down rpg with a sparing mechanic, allowing you to avoid fighting the Monsters you encounter. Here's a review by IGN that I feel explains Undertale perfectly.
Story
The story of Undertale follows Frisk in their journey through the Underground in an attempt to escape. Frisk falls into the Underground, where the Monster race was sealed long ago. Throughout your journey you'll meet many different Monsters and you'll have to choose whether or not to spare them. This choice will impact the ending based on who you spare and who you kill, as well as your level.
Graphics
The graphics aren't anything special. The game uses pixelated sprites and textures, and it uses them well.
Soundtrack
Undertale's soundtrack really sets it apart from most other games. It fits the environment perfectly and overall has some of the best music I've heard.
Difficulty
Undertale isn't too difficult if you play one of the normal routes, but if you do the genocide route you'll be faced with the hardest bosses in the game.
Pricing
Undertale is $10 at full price and will occasionally go on sale for as low as $3. With it's many different routes and fun gameplay, you can easily play it for many hours.