value parameter, variable parameter

Hello

I`m trying to understand what this is all about "value parameter" and
"variable parameter" in procedures.

Let`s say I have some procedure: proc1(a,b :integer)

and some operations on a and b (maybe together with other variables x,y,z)
when does it affect the outcome of the procedure at choosing a,b as value parameter or variable parameter?

I`m really confused about this and hope you can help me!!!

Thanks
matter