I'm no Ishkur, but I do know a thing or two about music subgenres. I'll be glad to help.
Keep in mind: 1. Konami isn't the least bit particular about categorization, and there are many subgenres that sound funny, aren't actual subgeneres, or are even deliberately incorrect (Summer Vacation, anyone?), 2. a lot of subgenres are highly subjective and are defined at least as much by attitude and image as what they sound like (new age, visual kei, goth, grunge, industrial, etc.).
Examples given in parenthesis.
POP (A.K.A. POPS) - Go back to DDR, PoMu, IIDX, KBM, ITG, PIU,
and Jubeat! Mainstream-acceptable radio music that there is absolutely no getting away from.
(Haru Spring, Sunny Day Sunday, Now I'm Sure, Hanabi, Shiritori, E-mail Me, Moonlight Walking, Rouge No Dengon, Depend on Me)
ROCK - Just guitars, drums, and someone to play them. No pretentions, no prejudices, as pure as it gets. Called "classic rock" by those who find Nickelback absolutely disgusting for some reason.
(Chicago Blue, Smoke on the Water, Born to be Wild)
ROCK AND ROLL (A.K.A. ROCK 'N ROLL) - The music of Elvis Presley, The Beatles, Dire Straits, Huey Lewis And The News...heck, pretty much anyone worth anything in this business. Dance rock. Fun rock. Disco before everyone got sick of disco. Dependable, endlessly catchy riffs with drum accompaniment. Eternally appealing. Hey, you figure there's a reason the Hall of Fame is named after this?
(Dare?, Used to Rock 'n Roll, Moso Gakuen Ino-Koi Gumi, Used Cars, Electric Sun, Rockin' Paradise, Memories)
POP ROCK - What passes for rockin' rocker rock-rock on American Idol. What the radio DJs play when the sappy, soppy, treacly material becomes too unbearable even for them. It's pop, just a bit heavier on the guitar. And drum.
(Break Down, Himawari, Sailing Day, Tentaikansoku, Hikari E, Summertime Cruisin', Shippo No Rock, Brave!, Andante, Departure, Taisetsu Na Mono, Home Girl)
SURF ROCK - There might be a "proper" term for this, but this is what I always hear it called, so I'm calling it the same. Lower-key instrumental guitar music which brings up vivid images of white sandy beaches and sparkling blue oceans*. Distinctive "tumbling" riffs.
(The Endless Summer, The Adventures, Pipeline)
GOTH ROCK - Has a moodier, bitterer edge than mainstream rock (although a lot of it is great listening in its own right). Close cousin to visual kei, which is largely a Japan phenomenon.
(Geko Cho, Hotaru, Romance, Agnus Dei)
HARD ROCK - Energy. Passion. Fury. Adrenaline.
Hard, man.
(Fire, After a Hard Day, Dream Again, Heaven Inside, Scenario)
PROGRESSIVE ROCK - Whether complexity, length, or drama (the more cynical would say "pretentiousness"), there's more work involved in this than ordinary rock. If there is such a thing as "ordinary rock".
(Primal Soul, DD series, Namida No Regret, Cockpit, Kaleidoscope, Tamayura, Timepiece Phase II)
SWING - That weird craze from the late 90's that you still don't have the slightest idea what it was about. Peppy, somewhat repetitive brass instrumentation, light guitar/drum sounds. Big billowy skirts and suspenders. Dancing a must, complete with perky ladies in said big billowy skirts being held upside down a minimum of three times every routine.
A close cousin of jazz.
(Ringo To Hachimitsu, Heaven Is A '57 Metallic Gray, Fireball, Boogie Train '03, Akai Suzu)
SKA - More or less fast-paced rock, but with horns. Trumpets, saxophones, whichever. Made famous by Five Iron Frenzy, then made incredibly confusing by No Doubt.
(Swimmers, Mikeneko Rock, Get It All)
JAZZ - Catchall term for various traditional American instrumental styles. Uses piano, acoustic guitar, trumpet, saxophone, drum, triangle, cowbell, everything but the bathroom sink.
(Jazzy Cat, One Piece Blues, Joker, Tocatta, Take My Hand)
R&B - The genre that uses the word "diva" way, way too much. Sassy, spicy, sultry, and various other adjectives beginning with s. (No, not
that one...get your mind out of the gutter.) The music most associated with sexy black female dancers.
(You Can't Hurry Love, Reaching For The Stars, Kiss Me Goodbye, Doki Doki, Termination)
PUNK - Music that's so rebellious, it can't stand
itself. Full of anger at all the injustices of the world. Such as Rock Revolution.
(Lucky?Staff, Circle Coaster, Wasuremono)
POP PUNK - Hold on...isn't that a contradiction in terms? You'd be surprised. Similiar punk sound, but covering far less controversial subject material.
(Futari Wa Love Love, Silly Girl, Honjitsu Wa Seiten Nari, Guitar Rock, Hitoribochi)
AMBIENT POP - Chill-out music. Slow, peaceful, gentle, and otherwise a complete radical 180 from what you'd expect from Gitadora.
(Hitomi ~suisou no naka no mirai~ [sumpthin' like that], Akogare, Wish)
HEAVY METAL - "Oh my god, he's screaming! This isn't singing, it's screaming! And that music is so loud! Loud, loud, loud! Screaming! Loud!"
(Jet G Crew Zekkocho, Family Restaurant Bomber, Voiddd)
PROGRESSIVE METAL - Less shouty, more music-y than most types of metal, and usually fast-paced. Supposedly more "complicated" than usual.
(Dragon Blade, Atalante, Ring, L.A. Rider, Rise, Black Sheep)
POP METAL - No, not even metal was spared the insidious fingers of mainstreamitude. Love and living and having fun and heartbreak and caring...in metal form.
(Bad Medicine, Liar Liar, Magic Music Magic)
THRASH METAL - Heavy metal on some cliched chemical stimulant.
(Handsome Kyodai Made In Japan, Mikan No Uta, Sanagi, Mad Blast)
ROCK OPERA - Still lots of guitar and drum, but with a more "oldskool" (as in several centuries ago) edge. Often "borrows" from famous classical pieces.
(Classic Party series, Seiron)
BUBBLEGUM - Made eternally (in)famous by
one stupid Aqua song, which is like Baha Men being known for...oh, wait. Anyway, this is the "super cute" side of music, which invariably draws comments like "little kids" and "diabetes" and "lolicon".
(Milk Candy Citrus Drop, Luvly Merry-Go-Round, Sakuranbo, Bayside Baby, Sobakasu, Star No Kobito)
TRADITIONAL JAPANESE - Anime music. Excruciatingly traditional instrumental songwork that was old
during the Tokugawa era. Some of it is pretty cool, actually. Any game with Japanese music is going have at least a little of this.
(Fuma Tenku, Uki Fune, Sandogasa Ponta series)
LATIN - From our friends south of the border. Or several degrees south of the direction you'd normally take to get...never mind.
(Misirlou, Passion, La Arena Roja, Tierra Buena)
WORLD - Songs that are
so out there, they don't have
any game to go back to. Sometimes called "ethnic" music.
(Balalaika Carried by the Wind, O Jiya, Tara Yum Kahan)
Irreverent? Sure. So is Ishkur.
P.S.: The only Bemani title that's all
anything is Para Para Paradise. Let's not talk about that anymore.
* Is it just me, or are a
lot of Gitadora videos set at a beach and/or ocean?
Genre listings for GF / DM songs? 
