If someone can explain about programatically generating session ID to login into sales force.

As this is something that we need to acheive by allowing user to logon to our internal website and then show a link to salesforce on our site. Once a user clicks that link we need to take the user to sales force directly without logging on to sales force.

Is there any other way to achieve single sign on ? Link to any sample code or examples on which could help in Single Sign On.