In a nutshell a SSH tunnel is a means to transfer unencrypted data over an encrypted channel. Typically the data is a TCP/IP protocol like email, web or a database server and the source port is the port used by the service on the remote computer. For example I've included a link to tunnel mysql over SSH. The source port is the port that mysql uses which is 3306. Depending on the version of putty you are running the setup pages may not look the same. The local port can be the same or different depending on what services the local computer is running but typically an unprivileged i.e. >1024.
If you wanted to tunnel a web server then the source port would typically be 80 and the local port be anything >1024 so lets pick 8080. Once connected, in your web browser you would use the URL localhost:8080. You should now see the default page of the remote web server.
http://realprogrammers.com/how_to/se...ith_putty.html
