How to copy files from one machine to another using ssh

I am using a Linux (CentOS) machine, and I have already connected to another system using SSH. Now, my question is: How can I copy files from one system to another system? Suppose, in my environment, I have two system like System A and System B. I'm using System A machine and some other using System B machine. How can I copy a file from System B to System A? And, copy a file from System A to System B?