A train is called a high speed train if its maximum speed in normal operation is at least 200 kilometers per hour (125 mph).

Maglev

Despite decades of development, magnetic levitation (aka maglev) technology permitting speeds above 500 km/h still remains to be commercialized. A first baby step was taken in January 2003 when Shanghai's 60-km Pudong Airport Transrapid express link opened; China has already signed a contract to extend the track some 300 km to Hangzhou and is considering building the 1500-km Beijing-Shanghai link as maglev.

Dedicated Track

"Real" high speed trains run on dedicated track at speeds of up to 300 km/h (186 mph). There are three families of high speed train technology, although the recent trend in Europe has been to build hybrids like the ICE 3 and Eurostar that can deal with multiple signaling systems.

Tilting

Tilting trains can tilt into a curve to allow higher speeds without reducing passenger comfort. They run on minimally modified normal track and can reach speeds of up to 250 km/h, although 200 km/h is the usual maximum. Tilting technology has also been added to dedicated-track high speed trains, namely the ICE-T and the TGV Pendulaire.

References

http://www.o-keating.com/hsr/
and a tip o' the hat to mawa