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, heightening the tension.
Hard trance declined in popularity in the mid-2000's, and many of the DJs in the style began to focus on hardstyle instead.