Basically, the shorter and higher the bar/stem setup, the easier it is to pull up the front end while moving forwards. This is useful for more streety moves such as manuals, bunnyhops and spins. For more TGS type riding you tend to want a longer, lower setup which helps with taps etc but also gives you more room for wheel swaps etc. To give you ball park figures, a streety setup would be a stem less than 100mm, probably with a good 15-20 degree rise combined with fairly high rise bars (50mm+). A more traditional trials setup would involve a slightly longer stem, probably around the 110-115mm mark with less of a rise (10-15 degrees) and with standard trials type riser bars (think TrialTech/Monty etc).