Genre: It's the template for grunge
To this day, I instinctively try to type "never mind" as a single world.
This is the album that killed hair metal. Perhaps as a reaction to the New Wave and party music of the 80s, the music of the early 90s all took itself very seriously. It was also the era of gangsta rap, after all. Gen-X-ers were the new youth culture and they were angsty and ironic this record in particular spoke to them. It was rough and raw but somehow also pop-friendly and glossy. It was angry but also fun--it was basically punk music, but it was slower and grooved a little. The lyrics were vague and weird and didn't seem to be about anything, but they were also poetic and beautiful in their own way. (And then you actually find out what songs like Polly are about and it sort of blows your mind.)
It has just so many good, enduring songs on it. The singles Smells Like Teen Spirit, Come As You Are, Polly, Lithium, and In Bloom get radio play to this day. Late-album fare like Drain You, Something In The Way, and On A Plain still hold up incredibly well and would get a revival on the bands MTV Unplugged In New York album. Stay Away showed up on the DCG rarities compilation under its original title Pay To Play (This an album that has no reason to exist, but it does exist, and it even had a single: Counting Crows' Einstein On The Beach (For An Eggman).) The hidden track Endless, Nameless gained notoriety for how batshit insane it was that someone would record 7 minutes of instrument-destruction like that, let alone include it on an album. Even the filler tracks, Breed and Territorial Pissings, are catchy.
I'm also a big fan of (the only song I haven't named yet) Lounge Act, largely because it's a nice little showcase for bassist Krist Novoselic. Drummer Dave Grohl and singer/songerwiter/guitarist Kurt Cobain are--rightly--heralded for their talents and contributions, but Novoselic is a bit of an unsung hero. His bass work was never flashy, but in a genre where bassists are assumed to be playing straight root 8th notes, he was always finding something interesting and functional to do. Years of playing with Cobain had knitted them into a tight unit, and I think it's a little under-appreciated how integral he was to Nirvana's sound. This song lets him show off a little.
Further Listening: Cobain's suicide in 1994 cut short Nirvana's career. In a way, that meant that they never had a chance to get stale or tired, and it paved the way for Dave Grohl's decades-long success with Foo Fighters, but it also means the Nirvana catalog is limited to three proper studio albums and a B-sides collection. Their indie debut Bleach, is rough and features a different line-up. Incesticide, the B-sides album, is similarly difficult to listen to, although it did give us Sliver and Aneurysm, which are fun. Their final studio album was In Utero, an intentional step away from the pop sheen of Nevermind. It's less accessible, but still has some amazing tracks on it and a healthy dose of irony, as evidenced in titles like Milk It and Radio-Friendly Unit Shifter. MTV Unplugged In New York is a concert album, and almost half of it is covers of other artists: the Vasolines, Lead Belly, and David Bowie, and then three Meat Puppets songs that feature Curt and Chris Kirkwood of Meat Puppets. It's an odd duck of an Unplugged record, but still quite excellent. I never did get into their other live album, From The Muddy Banks Of The Wishkah. Their hidden track from the No Alternative compilation is worth seeking out, titled variously Sappy, Happy, or Verse Chorus Verse. Finally, there's the song You Know You're Right, which was the last song the band recorded while working on a fourth album that remained unreleased until 2002 because of legal disputes between the surviving band members and Cobain's widow Courtney Love.