By default VNC is not encrypted, and therefore is not as secure as some people may want. A common workaround for this is to have SSH establish a secure tunnel either directly to the computer you want to control, or to a computer on a "trusted" network. This will be done by using the "ssh" command in the terminal to create a secure port-forwarding session with a remote computer, and then connecting to the forwarded port using "Screen Sharing". Keep in mind that the following is a bit complicated to understand, but overall is a fairly simple process; however, if you are not prepared for setting this up then you can skip this section and continue exploring screen sharing without the added security. The first thing you will need is to ensure the SSH server is running on the computer to which you are creating the "tunnel". To do this, if the computer is a Mac go to the "Sharing" system preferences and check the "Remote Login" option. Then launch the Terminal on your current Mac to enter commands for creating the secure tunnel to the ssh server.
Read more: Tutorial: Screen Sharing in Leopard (Mac OS X 10.5): How it works and how it doesn't | MacFixIt - CNET Reviews