I have opened a port in my firewall to communicate from server to client and vise versa. If the client is behind a firewall at my client do they need to open the same port in their firewall... And if so... can somebody please explain.
That depends on the firewall. Many firewalls, like the Windows XP firewall, just block incoming connections. If this were the case at your client, he could make a connection to your server no problem. Other firewalls, like ZoneAlarm, inform the user if a program attempts to make an outgoing connection. If this is the case at your client's site, he will need to configure his firewall to allow a connection.