ssh_config is the configuration for the client. You use this file from a system when you connect to some other machine using ssh.
sshd_config is the configuration file for the server, the daemon that runs in your system to accept incoming ssh sessions.
The server file is read when the server is started. If you make changes to it, you need to restart the daemon to pick up new changes. Not so sure on the client, I would bet it is read any time you try to connect to a server though.
If you need more information, I would send you here --> http://www.openssh.org/