Can you base salaries on evaluations?

One of my friends told me that you cannot base salaries on evaluations. I doubted it but I thought I'd ask. The thing is that I worked at this job the exact same time as another friend and she is making a little more than I am now. The evaluation policy where I work is ridiculuously biased. What is the exact policies and what can I do to fight it if salaries cannot be based on evaluations?