The premise is Toy Story-like: the heroes, villains, monsters, and other assorted creatures you see in a video arcade are actually alive and conscious inside the box. When the arcade closes, they commute down the game's power cord to "Game Central Station" where they schmooze and engage in other leisure-time activities. (I bet they don't play video games themselves, though.)
The titular hero of this movie is the villain of the game "Fix-It Felix": as Ralph attempts to destroy an apartment building, Felix is there with a magic hammer to repair the damage, defeat Ralph, who invariably winds up getting tossed off the top of the building to his demise.
But Ralph is a villain with a heart of gold, and he's getting tired of the rejection and stigma attached to his role. Could he find happy acceptance in a different game? This kind of thinking is taboo among the video beings, but Ralph tries it anyway. His efforts produce multiple unintended consequences threatening the entire collection of arcade machines.
It's very inventive and fun—probably even more fun if I'd played more recent video games—and the voices are near-perfect: John C. Reilly as Ralph, Jack McBrayer as Felix, Jane Lynch as a no-nonsense military commander from a shoot-the-aliens game, and Sarah Silverman as Vanellope, a "glitchy" character in "Sugar Rush", a racing game aimed at kids.
I recognized Q*Bert, though.