Hard Trance falls into two distinct periods, but both have their birthplace in Germany. The original style (which became popular around 1993-94), was typified by a fast (+150bpm) tempo along with strong acid lines and prominent melodies. Popular acts around this time were Sunbeam, Komakino and Jeyênne.
This progressed up until the late 90's, when a new style of Hard Trance emerged thanks to the likes of DJ Scot Project & Kai Tracid. This new style brought the tempo down into the 140bpm range, and focused more on Euphoric Trance in construction. The kick drums & basslines were exaggerated, bringing an aggressiveness which separated the style from standard Trance. Also, producers such as DJ Scot Project pioneered extended breakdown / buildups, hightening the tension.
It ceased being popular in the mid-2000's.