With underground rap changing so quickly, it's hard to strictly define all the various underground rap subgenres. Thanks to the Internet and accessible technology, anyone can push forward a genre like underground rap with the few clicks of a mouse.

As the rate of musical change continues to accelerate, new subgenres of underground rap are born, die, and change every few minutes, it seems. Many older established subgenres are becoming more popular, even as fans have a hard time defining them exactly.

One example of a couple of underground rap styles that overlap are horrorcore rap and acid rap. Acid rap is probably the lesser-known and more contentious of the two underground rap styles. Both underground rap subgenres share certain stylistic elements, sonic approaches, and lyrical content.

But not all acid rap artists are horrorcore rap artists, and vice versa. To understand the difference between horrorcore rap and acid rap, we can first look at the history of both of these underground rap genres. 

Horrorcore rap and acid rap developed around the same time, largely in the early '90s, though horrorcore rap can be said to date slightly earlier, to the late '80s.

One of the first horrorcore rap groups was the Geto Boys, who were known largely as a gangster rap group, but started weaving in horror movie-style imagery into their music. Their song  "Chuckie," about the infamous murderous doll form the Child's Play movies, was considered one of the first horrorcore rap songs.

Much of the early horrorcore rap violence was not particularly realistic and was inspired by the fantasy of movies and television. Soon after the Geto Boys came some of the earliest artists who specializes almost exclusively in horrorcore rap, like Kool Keith and the Flatlinerz in New York.

Around the same time arose so-called "acid rap," which was the tag preferred by the Detroit artist Esham. He picked up many of the budding themes of horrorcore rap in his lyrics, but his sonic palette was a little broader.

The "acid rap" term was meant to nod at psychedelic rock, which often influenced the actual sound of his music, which fused typical hardcore hip-hop beats with metal and other elements. Esham also went beyond just talking about horror in his lyrics and talked about broader issues like paranoia and drug use.

Esham influenced many artists in both subgenres. Some picked up largely on just the acid rap part, focusing more on the psychedelia and trippy, drug-influenced vibe.

Other artists have later been tagged acid rap just for their unpredictable, crazy sound, even if they don't trace a direct lineage to Esham. For instance, some underground rap acts from outside of Esham's Detroit circle who have been called "acid rap" are Deltron 3030 and Cage.

Meanwhile, the hallmark of horrorcore rap is the lyrical content. Horrorcore rap may draw the sound of its beats from some acid rap, but horrorcore rap is largely about over-the-top, cartoonishly gory stories.

Some major artists have been influenced by both, like one of the most infamous underground rap groups of all time, Detroit's Insane Clown Posse. Taking the persona of murderous clowns, the group was so outrageous that it took horrorcore rap out of underground rap circles and into the mainstream.

However, they claimed Esham as a major influence, and they have always been among the most creative of the horrorcore rap acts, often incorporating a creative acid rap style to their beats.

Still, despite all this discussion, you shouldn't get too wrapped up in splitting underground rap hairs. Horrorcore rap, acid rap, whatever -- enjoy the music you enjoy and don't get caught up in labels.

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Insane Clown Posse is hands-down the most well-known underground rap act drawing from both horrorcore rap and acid rap. The group has spawned an entire subculture known as juggalos, and which runs its own record label, Psychopathic Records.

To learn more about this seminal  horrorcore rap</a> group, visit the official Insane Clown Posse web site at http://www.insaneclownposse.com.

Music is a fluid thing, of course. Genres change and morph over time, especially in the fast-moving world of underground rap. These days, there are any number of countless underground rap subgenres, and it's hard to keep track of them all, even for devoted fans.

With smaller underground rap sub-scenes including everything from "crunkcore" to "backpack rap," it's enough to make an underground rap fan's head spin.

That's especially true since many underground rap subgenres overlap in style and content. For example, a couple of genres who share some, but not all, artists and fans are horrorcore rap and gangster rap.

Both of these underground rap styles favor a hardcore lyrical approach and aggressive beats. But they're not identical. So what is the difference between horrorcore rap and gangster rap? First, a little background on each subgenre individually.

Gangster rap appeared slightly earlier than horrorcore rap, largely in the underground rap scene of the West Coast. Early artists included Ice-T and Schoolly D (who was actually from Philadelphia), and then a little later, N.W.A.

Gangster rap picked up on earlier so-called "hardcore rap," which boasted more confrontational lyrics and aggressive beats than the earliest rap, which was more party-oriented.

Like the name implies, a lot of gangster rap focused on recounting tales of street and gang life, and songs were unabashedly full of violent, but realistic, imagery. Other pioneering gangster rap artists included Too Short and Cypress Hill on the West Coast, and New York's Boogie Down Productions on the East Coast.

Horrorcore rap developed slightly later than gangster rap, although both of the genres' development has overlapped over the years. Horrorcore rap first appeared in the very late '80s and early '90s with groups like the Geto Boys.

Though the Geto Boys were considered at that point to largely be a gangster rap group, they began weaving in songs with horror movie-influenced imagery. Famously they wrote a song called "Chuckie," which, with its lyrics about the infamous murderous doll form the Child's Play movies, was considered one of the first horrorcore rap songs.

Where horrorcore rap distinguished itself was in its over-the-top take on violence. Much of this violence was not particularly realistic and was inspired by the fantasy of movies and television. Soon after the Geto Boys came some of the earliest artists who specializes almost exclusively in horrorcore rap, like Esham in Detroit and Kool Keith and the Flatlinerz in New York.

Just a little later arose one of the most infamous horrorcore rap groups of all time, Detroit's Insane Clown Posse. Taking the persona of murderous clowns, the group was so outrageous that it took horrorcore rap out of underground rap circles and into the mainstream.

While horrorcore rap and gangster rap share certain themes and subjects, they're still not totally the same. Some artists, like DMX and Eminem, have made both gangster rap and horrorcore rap. But the main difference is the degree to which they take their tales. Gangster rap lyrical violence is still rooted somewhat in reality, even if it's composed largely of boasts.

With horrorcore rap, the understanding is that nothing anyone is talking about is meant to be taken literally. This gives horrorcore rap, in a way, many more creative possibilities in underground rap. Because the genre doesn't need to be seen as "real" as gangster rap, horrorcore rap artists can be as disgusting, cartoonish, or ridiculous as they want to be.

They can also draw from a more diverse sonic palette. Whereas gangster rap often requires stripped-down beats, many horrorcore rap artists draw from a wide spectrum of the musical map. Horrorcore artists often incorporate elements of rock, metal, and a number of other styles into their sound.

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These days, gangster rap has started to wane fro the mainstream a little bit, while horrorcore rap continues to gain velocity in the underground rap scene and beyond. One of the main groups helping make this happen is Insane Clown Posse, which has spawned an entire subculture known as juggalos, and which runs its own record label, Psychopathic Records.

To learn more about this seminal horrorcore rap group, visit the official Insane Clown Posse web site at http://www.insaneclownposse.com.