Loops in Batch files

Does anybody have a really clear concise description of how FOR loops work in batch files. Some examples would be nice as well. I've read lots of sites worth of info on them but I cannot seem to get them to work.

Here's what I am trying to do. I have written a batch file which detects the OS (95, 98, XP) and if a certain CD is inserted in a drive (drives D through Z) using a rudimentary IF loop with an incrementing variable. First it checks option one and if the wrong errorlevel returns a variable is incremented
"set var=var1"
then it check option two and uses the same incrementing variable to tell what the correct answer is. This method works but is prone to errors due to its length, also non-standard drives (ZIP, empty CD drives, etc) tend to create message boxes and stall the loop. For the 30-odd variables I'm checking I use 200+ lines of code.
I believe, based on what Ithink I understand about them, that FOR loops would allow me to shorten this significantly as well as provide a way of eliminating some of the errors from non-standard drives.