Pokemon Online Server is a program in which people can create desired teams and test them out against other members without the hassle of making the Pokemon(training, ev traning, catching, ect) It takes the hard work out of it. Its a pretty popular program but if you dont already have it I recommend downloading it at http://pokemon-online.eu/
Now once you have it. You can join "servers" which are just like places where you can chat and battle with other people connected to this server. Our site(EPT forums) has its own. So we can battle within our website and not with strangers and make teams and test it out without all the hard work. If you would like to join it its called Elitepoketrainers in the server list once you go online.