This is a big topic, here is a little:
LST(Low stress training)-bending of the plant. why, because instead of light's lumens being discarded on the top colas you now have a plant that has been bent horizontal and lumens are more = across the entire main stem of the plant.
HST(high stress training)-
any cutting of the roots, stem, branches. why, Look up SOG(sea of green), SCOG(screen of green).
essentially, you train a plant to fit with your grow room, produce more bud per Sq. foot, or you might just want to expirement.
found this website, it should give you a better understanding.
http://www.icmag.com/ic/showthread.php?t=104379
peace out, and enjoy playing with the plants.