Duke Nukem 3D is a sci-fi first-person shooter created by 3D Realms and published by GT Interactive Software for PC in 1996.
The game is the sequel of Duke Nukem and Duke Nukem II, two shooters/platformers created by Apogee Software. 3D Realms was a new division of Apogee, dedicated to 3D games, and Duke 3D was their second title, after Terminal Velocity.
The game is considered a real classic and one of the games that made the genre of 3D first-person shooters so famous. Probably the only example more famous than Duke Nukem 3D is Doom, created three years before. 3D Realm's game took a lot of inspiration from id Software's masterpiece, but is superior in some aspects, including gameplay, puzzles, interaction and destructible environment. Duke 3D also has more story and gives players the ability to jump, crouch, and fly.
There's a lot of discussion about which game is better, but you don't have to choose, just download all of them and have fun!
Duke3D's the biggest of the later pseudo 3D game crowd which "placed itself" as a light hearted alternative to Quake. (Both games had a mutually exclusive territory so it was kinda lame that they got pitted against each other in those magazines of yesteryears.)
While I personally do not like most of the enemy designs due to their poor quality the game is overall one of the few mega hits of the genre and the most famous Build game. Where it really shines is in the level design - especially episode 1 has some of the finest entries in sector driven games(e.g. "Death Row", "The Abyss").
Episode 2 is not my cup of tea, but relative to its space theme, is good and has one really outstanding map - "Dark Side".
Episode 3 seems better, as the city theme is the strong side of Duke, but can't touch the first one. There's still great nuggets like "Flood Zone" which provide an awesome setting to explore.
Plutonium Pack added a fourth episode(and a new enemy) that never really stuck with me. The map's are well made with some strong entries, e.g. "Duke-Burger", but to me the later maps do not carry the magic of the ones featured in former episodes... Duke, like Doom, was first introduced as a "three part-er" so it's hard for later add-ons to be "seamlessly included" and accepted.
There's a myriad of fan made stuff out there and even a new, fifth episode, available. Add to that the ports of Duke to the consoles of the 90ies(+modern ones) and one got a LOT of content to dig into.