Baldur's Gate is a series of Role-Playing video games and the name of the first release in the series. The first game was released in 1998, and changed the RPG genre forever. Seriously, before that people thought that Diablo was an RPG. Then they realized it was just an addictive hack 'n' slash click-fest. Baldur's Gate provided not only challenging and tactically interesting combat, but also an intriguing storyline that actually places your character in the plot, which is affected by your actions and decisions. Prior to Baldur's Gate, most RPG's introduced your character like the Stranger from High Plains Drifter.
Tales of the Sword Coast was released as the expansion.
Nobody thought it could be better until Baldur's Gate 2: Shadows of Amn came out in 2000. Gameplay was even better, story just kept getting better, and the best part was everybody knew an expansion was coming.
Finally The expansion to SoA came out entitled Throne of Bhaal. It is the final installment of the series. Beating the game gives you a combination of incredible accomplishment and also terrible depression from the realization that never again will you experience an RPG like it. Now we must all make do playing it over again and fantasizing about it while playing Dragon Age.
Blizzard has yet to release Diablo 3 ten years after Diablo 2 because they want to be absolutely sure that there won't be another Baldur's Gate installment that will blow them out of the water again.
Baldur's Gate is the perfection that all other RPG's are measured against. And they all fall short. Dragon Age came close, but only close.