Dichlor is typically a powdered form of stabilized chlorine (which means it has CYA in it) and Trichlor is in puck form.

As far as switching between Trichlor and bleach ----- once you get your cya level where you want it, it is pretty stable and you shouldn't need to use any form of stabilized chlorine any more. If you typically take a vacation during the summer and are away from home for awhile with nobody to add bleach for you, some people like to give themselves a little wiggle room when initially bringing the cya level up. In otherwords, keep it a little on the low side (maybe 30-35ish) so that when you use pucks while you are away on vacation, your cya level doesn't get too high. Just something to think about. I like to keep my cya around 40-45. When I'm away on vacation, I have somebody who will throw some bleach in every couple of days for me, though.