I need a place to help me think about what music genres mean while I try to standardize my mp3 collection and the genres I use for the record project. So here are my ideas. Fight me in the comments if you want. This is all subjective so neither of us are right.

Alt Country

Country music played by ex-punks/hardcore kids. Sure, they've got banjos and mandolins and sing sort of sad songs, but they also chewed on a lot of glass and yelled a lot in their old bands.
Examples from my collection: Chuck Ragan, Dave Hause, Rocky Votolato

Alternative

1990's weirdos doing rock music things. Contorted vocals and distorted guitars, but generally standard rock format. Not grunge, but not punk or pop. Purposefully strange, usually.
Examples from my collection: Hammerbox, Goo Goo Dolls, Pond

Children's Music

Music for kids, silly.
Example from my collection: A Sesame Street Celebration

Classic Rock

Any old wankers from before 1980 who really shredded the speakers of your dad's Camaro in high school.
Example from my collection: The Who

Classical

I put things played by a full orchestra into this genre. Soundtracks usually fit here and other things made by composers instead of rock stars.
Examples from my collection: Dvorak's New World Symphony, Sacre Du Printemps, Star Wars Soundtrack

Comedy

Music and other stuff whose main goal is to be funny. Wide ranges of genres fit here, but I'm not just talking about a goofy song by a normally serious band, I'm talking whole records aiming at laughs.
Examples from my collection: Patton Oswalt, Strong Bad

Electronic

Music from the future, made on digital instruments. No rave dance stuff, but kinda weird space music with keyboards, theremins, drum machines, etc.
Examples from my collection: well, I was going to put Octopus Project, but I don't actually own any physical copies of theirs, for some reason.

Emo

I don't use the purist definition (see "Emocore" below), I'm good with the second wave definition and third wave bands. Sad and poetic, quiet and loud, but still melodic and sad in a pretty kind of way.
Examples from my collection: American Football, The Hotelier, Sunny Day Real Estate, Texas Is The Reason

Emo Punk

This is where I'm doing nonstandard things, probably. No one uses this genre but I find it fits a lot of bands that aren't quite emo and aren't quite punk. It's like fast emo with more standard song structures. A little less poetic and melodramatic and a little more driving and upbeat. But still sad.
Examples from my collection: Alkaline Trio, Braid, Jawbreaker, Spraynard

Emocore

Early emo bands were just sad hardcore kids. They couldn't help screaming on their songs, so that's what they did. More harsh than second and third wave emo bands and more extreme dynamics changes and tempos. It's pretty good, but you can't listen to it all the time...
Examples from my collection: Amber Inn, Still Life... I should probably own more physical examples of this.

Experimental

Sometimes, you have to wonder if this is even music. It's waaaaay out there. Weird instruments, tempos, song structures, people, etc.
Examples from my collection: Bartholomäus Traubeck, Beast, Please Be Still, Efterklang

Folk

Legit folk music. Like the stuff that people just play on their porch-equivalent after a long day in the fields or whatever. Regular-ass people doing music stuff. Got some popularity in the 60's with hippies, but I try to avoid the hippie stuff.
Examples from my collection: John Denver, Simon & Garfunkel, The Lark In the Morning

Folk Punk

Punks love authenticity, and folk music is pretty authentic. So they do their own take on it. Sometimes this can be confused for alt country, but it's usually not as twangy and banjo-y.
Examples from my collection: Billy Bragg, The Cave Singers, The Smith Street Band

Grunge

Late 80's - Early 90's subgenre of punk. Sludgy and slower, with metal influences and local, artisanal flavors. You know all these bands.
Examples from my collection: Mudhoney, Nirvana

Hardcore

There's a wide range of things called hardcore. From 80's hardcore fast punk to 90's straightedge hardcore, I don't try to break them up. If you had a crappy day at work and need something to power your 20 mile evening bike ride, you're going to pick this genre and then maybe pick up some pennies in the pit or just pedal like a circle pit.
Examples from my collection: Botch, Chain Of Strength, Minor Threat

Hip Hop

I don't have a lot of rap and hip hop, I'm afraid. I dabbled in hip hop in middle school but the stories on those tapes were not my story and singing them made me feel like an impostor for feeling tough singing them. So I never really buy a lot and combine all the various forms of this type of music under this genre.
Examples from my collection: N.W.A., Macklemore, Yeastie Girlz

Indie

This is a bit of a catch-all genre filled with a very wide range of white person music from about 1999 to today. Prototypical indie music always feels very much like a soft focused lens flare at sunset. Sometimes it's a little overly nice, but sometimes it can give you feelings. Most of it is forgettable, but some of it is good!
Examples from my collection: Damien Jurado, Frightened Rabbit, Mates Of State

Math

Sometimes overlaps with post rock and metal a bit, but goes farther than either. Takes the rock genre and makes it a composition. The goal is to show off your technical ability with strange time signatures, tunings, tempos, and guitar effects. Mostly instrumental, in my classifications (see "post rock" below).
Examples from my collection: Bronze Fawn, Don Caballero, The Mercury Program

Metal

A thumb, index finger, and pinky finger are all you need. This genre spans from 80's hair metal up to present day versions of metal bands. Anything with a bunch of long hairs planning out guitar solos and being kind of scary, but in a very adorable way.
Examples from my collection: Helms Alee, The Holy Mountain, I guess I don't have that much metal (and no 80's hair bands? hm.)

Pop

You've heard these bands on the radio. High production values, big labels, and ABABCAB song formats for days. Or at least aspirations of such.
Examples from my collection: Cyndi Lauper, Phil Collins, Stevie Wonder

Pop Punk

I include early 90's east bay pop punk and early 2000's pop punk/emoish stuff in this category. It's sort of a strange group, but the word can mean two things. I take it as not-scary punk rock. Either it is silly songs like east bay pop punk or relatable songs like emo pop punk. Everything is polished or nice or both. And usually the songs are about girls (or relationships) and include woah oh ohs at some point.
Examples from my collection: The Bouncing Souls, Green Day, The Mr. T. Experience

Post Hardcore

Sort of a halfway mix between emo punk and hardcore. Rough around the edges, but not in your face about it. Some technical interestingness but not big hardcore stops and starts. When you sing along you put your finger guns in the air, but you wouldn't necessarily know any hardcore dances for it. It's in between a few genres, but there's some of my favorites in here, so I break it out.
Examples from my collection: Boy Sets Fire, Hot Water Music, Small Brown Bike

Post Rock

Math rock with words. Strange time signatures and weird guitar effects, but there's someone magically putting lyrics over the top. Most of these songs are described as "angular" or "jarring."
Examples from my collection: Engine Kid, Joan Of Arc, Slint

Power Pop

If pop stars try to do punk, I call it power pop. Or if it's just a little too far past pop punk, it could fit in the power pop genre. Maybe it's cleaned up a bit too much or it's a little to close to standard rock songs. Like it is trying to get on the radio, but still wants to wear its leather jacket.
Examples from my collection: Big Drill Car, Smoking Popes

Punk

Gritty and kind of gross. Or fast and badly recorded. It's a bit scary and often political rather than personal. From the 70's to modern punk, I cram all the roughest, toughest stuff in this genre.
Examples from my collection: Bikini Kill, Crimpshrine, The Clash, OFF!

R&B

I've always had a soft spot for 90's R&B from my youth. Smooth singers going all out for love songs and sad songs. I don't own too much, but I wish I had a stack of hits from that era to DJ someone's wedding with or something. Yeah.
Example from my collection: Brandy & Monica

Ska

We are talking first- and second-wave ska here only. Horns mandatory. Black and white checkerboard patterns a 100% requirement. Suits, ties, upbeats and 1, 4, 5 until the dance floor crumbles to dust. Pick it up!
Examples from my collection: Desmond Dekker, The English Beat, The Specials (kind of)

Ska Punk

Third-wave ska bands, here's your place. You don't even have to have horns to be ska punk. Faster, moshier, and, unfortunately, whiter. I spent a good chunk of the mid-90's skank-moshing to these bands, for sure.
Examples from my collection: Less Than Jake, Operation Ivy, Slow Gherkin

Swing

Ha! Remember those six months where people were getting into swing music? Big band sounds and big ol' pants or some such. Well, I may have bought a thing or two during that time. It's kind of fun, you have to admit.
Example from my collection: The Max Weinberg 7