Hardcore is a relatively vague term used to describe several styles of relatively fast
electronic music. In the late 1980's,
house was
disco's successor. DJs in the UK initially sped up
acid house records from Chicago and by the early 1990's, it became a genre of its own.
From the UK, artists like SL2 and Acen merged elements of house, breakbeat, and reggae. This type of sound would later speed up and spawn jungle and drum & bass. Also from the UK, songs like the Prodigy's Charly and Alpha Team's Go Speed Go defined the lighter fare. Belgians were also responsible for novelty fare like LA Style's James Brown is Dead and Rotterdam Terror Corps' Poing. However, this UK style would eventualy develop into the full-blown cheese that is happy hardcore, while Rotterdam turned progressively darker and much more aggressive. In Germany, Alec Empire merged punk elements into the mix and the result was digital hardcore.
These sounds influenced artists in the United States, resulting in seminal works like Joey Beltram's Energy Flash, which was a heavy influence in house-dominated Chicago. A New York movement, influenced by the Rotterdam sound, created its own brand of gabba.
These days, hardcore is usually synonymous with gabba, but could be referring to the early 1990's UK sound, digital hardcore, or the happy hardcore subgenre.