J-Core is a hardcore techno style that emerged during the mid 90's and was popularized by labels such as Sharpnelsound and ALiCE'S EMOTiON. Specifically, J-Core is a variation of Happy Hardcore and Gabber. It's known for fast-paced heavy sampling, often high-pitched vocals and incorporates many "otaku" themes, such as anime, videogames or typical japanese pop culture. More broadly, the term is also used as an umbrella term for a wide range of hard electronic music featuring these themes, ranging from jungle/breakbeat to speedcore. For a long time there was no term coined for this kind of music, but when this style gained international fame the term was coined around 2005 and from then on this style of music was referred to as J-Core.
During the mid 90's, Japan had a rich underground hardcore scene, but at one point the Japanese scene began to divide with one movement sticking to the original european style of hardcore (such as Bass2 Records and Teikoku Records), and another moving towards a "geek culture" kind of hardcore (such as Sharpnelsound, Toy Label and Murder Yacht School). This second movement was very much about self-made doujin (fandom) releases, not bothered by copyright since it was a very underground niche culture at the time. As a result, many albums feature unlicensed remixes and in recent years some labels have had to shut down the production of these albums or face consequences.
During the 2000's record labels such as ALiCE'S EMOTiON, C.H.S and X-Treme Hard started to emerge which brought a more polished sound to the scene and started to become more mainstream as the years progressed. At this time the scene also gained international fame. Various stores sold the albums internationally and events across the border started to happen too (such as Hardcore Synergy at Anime Central).
Nowadays, many of the artists that made a name for themselves in the 90's and early 2000's are commissioned for official remixes of anime or videogames or have original songs released on videogames like Beatmania IIDX, but the unofficial scene is still going strong and releases hundreds of new fan-made albums a year at events such as Comiket or M3 (Mixed Media Market) alongside the professionally licensed albums. Despite the amount of releases though, it is still a very fleeting and underground scene.