all of you made some interesting points.

^ i bought this book last year, it's 'Beats, Rhytm, Life - The Lexicon of Hip Hop Music' - it's a fantastic book, for real. it was written in 2005 by guys who really are into the history of the whole culture. here's what he said [i'm translating it from polish^^]
Do you really need any knowledge to admire the art, architecture or music? I think no. This kind of knowledge might be even a kind of barrier - it leads to futher analysing, comparing and shit. What for? Music is like a pair of jeans - yesterday tight, today baggy, tomorrow - who knows? The thing is that you feel comfortable.
On the other hand - everything has to have a beginning. Everything's got its history. You cannot judge the present or foresee the future without knowing the history. Especially when we talk about hip hop music - a melting pot of years of slavery, jazz, racial issues, blues, discrimination, funk, Malcolm X, poverty, Black Power and blaxploitation. A dynamic, changing collage, which is basically a recirculation of the whole musical heritage of human race. Like Busta Rhymes told me once: "you can turn every music into hip hop, but you cannot turn hip hop into anything else".
In general there are 4 basic elements of the hip hop culture: graffiti, breakdance, DJ'ing and MC'ing [plus beatboxing, often forgotten but how important at the same time]. 2 of them - DJ'ing and MC'ing constituted rap music.
..and it goes and goes for the next 8 pages, forgive me for not translating them

anyway, on page 513 starts chapter about different styles of rap. this might be kinda funny, but i think it's what explains everything - the difference between Lil Wayne and Nas, between Talib and Soulja Boy, Eminem and Outkast or NWA and Guru. there's a short note about each style, i'm just gonna list them and write a sentence or two, explaining it and also name artist who represent each style.
Abstract Hip Hop: mostly instrumental hip hop, but also experimental projects - mixing rapping with poetry. DJ Krush, DJ Shadow, RJD2.
Dirty South: 'third' coast rap, rap from Georgia, Louisiana, Texas and Florida. Lil Jon, OutKast, TI, Arrested Development.
Crunk: 'party' rap, which originated from the Southern rap. Lil Jon, Ying Yang Twins,
Dirty Rap: sexual content. Goldfinger, Kool Keith
Christian Rap: religious. Gospel Ganstaz [LOL!!], 12th Tribe
Conscious Rap: which often has got political background, but it's about social problems. Arrested Development, Talib Kweli/Mos Def [Black Star], Common, The Roots.
Kid Rap: rofl - Lil' Bow Wow, Lil' Romeo

this shit's weird, but yeah, obviously it's a genre..
Eastcoast Rap: positive, afrocentric movement, somewhere between 80's-90's - Native Tongues [De La Soul, Jungle Brothers, ATCQ], Jay-Z, Eric B & Rakim, LL Cool J, Jeru The Damaja, Ultramagnetic MC's and so on.
Electro: mix of funk and rap. Kraftwerk, Planet Patrol, World Class Wreckin' Crew
Emo-hop: (blahahahahah, that cracked me up!!

) - a journalistic idea mostly, regarding to artists whose lyrics are every emotional and private. Atmosphere, Emanon, Sage Francis.
Freestyle: we all know that rofl.
Gansta Rap: same here.
G-Funk: close to gangsta rap, more chilled though. characteristic organs, bass and funk samples.
Hardcore Rap: you can use this term to describe different types of rap, i.e. gangsta rap. it can be used in terms of lyrics and beats. DMX, Method Man, NWA
Hip House: mix of rap and house music. Big Daddy Kane, De La Soul, Queen Latifah.
Jazz Rap: obvious.
Miami Bass: deep, pulsating bass, fast beat, sexual content.
Old School Rap: Grandmaster Flash, Kurtis Blow, Whodini.
Underground Rap: far from mainstream. it can be used to describe different styles, it pertrains more to labels than to actual lyrical content. ya dig?
Political Rap: ya dig?
Pop Rap: mostly the very mainstream like Flo-Rida, T-Pain n shit. you get it, right?
Alternative Rap: far from popular styles and beats, mixing rap with gospel, rock, reggae, folk, soul. The Roots, OutKast, De La Soul, Del Tha Funkee Homosapien.
Latin Rap: latin artists, i guess it's more about origin than actual content.
Rapcore: rap + rock.
Trip-Hop: british answer for hip hop. it originates from Bristol, so UK artists tend to call it Bristol Sound. jazz, funk, soul, electro mixed with rap.
Turntablism: turntables + mixers.
Westcoast RapGolden Age: 80-90's rap.
omg, a long-ass post, but i just wanted to make it clear. i agree with what that guy wrote. it's important though to understand that there's no strict border between each style. like for example, Jazz Rap and Eastcoast Rap, or Hardcore and Westcoast. artists can connect between styles, that's the beauty of music.
also, from my point of view you can use "hip hop" while talking about the music only, because rap is actually part of the whole culture. you cannot use "rap" while talking about the culture in general though. i hope you get what i mean, it's like every square is a rectangle, but not every rectangle is a square lol. but stating the difference between "rap music" and "hip hop music" is totally pointless for me. or inventing a totally new genre called "rap/hip hop" - for heaven's sake, what the fuck is that?..