Whey people register on a site the are often told to click a link in there e-mail, I need these feature in my website design.What is the principle behind it?

Hi,
You need to use Mail object to send e-mails to users. In the e-mail's body you need to provide a link with a QueryString(s) which opens one of the pages of your web-application along with parameters(QueryString). Using that parameter(s) which are sent to your application you can make decisions and run different codes.

I hope it helps,
Cheers.