(i'm new) Question about for statements (probably easy to most of you)

in the arduino tutorial, it shows us how to create a for statement like this- for(int x = 0; x < 100; x++) but it does not say what each piece of it does. in the variable x = 0, what is the number zero (0) for and how could i find/make it?
How do I choose the x < 100 either? does it just stay at < 100, or do I edit it to match my own code? What is it?
and lastly, it says x++, what does this mean? what is the difference between x++ and x--? How do I choose which one to use in my code?
Thanks very much.
Cmor