First, there isn't a set bpm for 16th notes -- this is the type of notes you're playing. Going with 4/4 time at 120 bpm you have 4 quarter notes per measure. After quarter notes you have eighth notes which is half a quarter note and then sixteenth notes which are half the time of eighth. I'll try to make a chart to help explain that.

whole note
|
half note half note
| |
quarter quart. quart. quart.
| | | |
8th 8th 8th 8th 8th 8th 8th 8th
(and then 16th, then 32nd, then 64th, etc...)

Shredding just picking fast (i believe) and can have legato runs. I usually include techniques like sweeping in this as well.

Hope this helps, and if I have said anything wrong then hopefully someone will speak up soon.