Completed a job, how does reputation work?

I've completed a job and myself and the client are happy with the end result. I've seen profiles that have rep from people doing jobs on the person that hired them and vice versa though we cannot seem to find out how the rep system works. We've gone though all the steps and hes paid me for the job I did but cannot figure out how to rep eachother.

Sorry if this is a dumb question and It's right in front of us but we have no idea, but with all the requirements for rep now it can help out to gain for rep for me because I'm new on the accepting job side of things here.