what the hell is this yu-gi-oh stuff I keep seeing?
It’s a collectible card game that has its own television cartoon series and accompanying assortment of video games.
It started out as a manga (comic series). The early stories, before the card game became central, are much better than the card-game based episodes.
Yugi Mouto is a shy, game-obsessed kid, who gets ahold of a ‘Millenium Item’ - which brings out Dark Yugi - a splinter personality, and, early on, a right bastard - and gives him access to the Shadow Games. The Shadow Games are games with very real, very unpleasant, consequences for the loser (mostly they go insane). Dark Yugi also kills (or at least serious maims) several people using more mundane (if unusual, and poetically just) games - like one involving a vial of explosive, or a lighter held above high-proof alcohol, etc. Dark Yugi usually only emerges when Yugi’s friends are harmed or in danger.
The card game is first introduced early on, but after Kaiba’s dispatched that first time, it goes back to ‘a different game in each story’.