J-core is a term used worldwide for Japanese-Core, referring to Dance and Happy Hardcore. Ususally songs sample some form of anime/film and have a high bass factor. J-core is often an underground scene and it is very hard to get hold of.
Following the trend of prefixing a genre name with J- to signify Japanese origin (eg J-pop, J-rock etc.), J-core is an abbreviated term for Japanese hardcore dance music.
Though many only associate this term with artists such as DJ Sharpnel who are well known for their very Japanese-flavoured anime/film sampling hardcore, the umbrella term can be used to describe many kinds of Japanese-flavoured hardcore, such as happy hardcore, freeform, speedcore and terrorcore.
This term can sometimes be used to describe hardcore punk music from Japan as well as just hardcore electronic/dance music.
Notable J-core artists include DJ Shimamura, Buzzmasta, M-Project, DJ Chucky, DJ Technorch, DJ Sharpnel and REDALiCE.
Personally I became interested in Japanese speedcore in particular after listening to "Betwixt & Between - 12 little fragments of silence"