comparison of data

[Cathy Ciret]

Hi
I,m a new user of GRASS,and I'd like to be able to compare maps (with
the same categorie values).Does anybody know how to do it?
I've got map of vegetation with ,for example 5 or 10 classes,and
I'd like to get the DIFFERENCE between those maps.It 's look
easy to combine or overlay map in Grass,but I just can't find
useful commands to make comparison...
Thanks .