tremolo picking is a technique for metal songs(Mostly). slayer, soulfly, slipnaut, fear factory, and even some older metallica songs use tremolo style picking. basically tremolo picking-is moving the pick up and down(this can also be called alternet picking). The difference between alternet picking and tremolo is that tremolo is picked very fast(Thats why you have to use both up and down strokes because you can only go so fast using down strokes),alternet picking is up and down strokes at any speed. tremolo picking is easy to learn but takes a while to get the triplets,and quadruplets accuret so just make up your own riffs thats usually the easiest way to get better. something like this
----------5--7-----------3--5-------------
----------3--5-----------1--3-------------
-000-000-------000-000-------------------
This basic riff is done in triplits-down up down up motion.