I know only basic PHP so I can't really give you many details but here we go.
First I see you got a form to get the URL. From there your going to need to look at the your going to need to find some way to get the info thats on that site. Most search engines use web crawlers; computers that go to the site and follow all the links and grab the data for you. But another way is to maybe to grab the source of the page. From there you can past that data into form and have it look for key meta tags. This page has
<meta name="description" content="Post reply" />
<meta name="keywords" content="PHP, MySQL, bulletin, board, free, open, source, smf, simple, machines, forum" />
Then your script will take the tags and well, store them. As you can see just by the tags this page is used to Post Replies and is for PHP, MySQL and so one.
As for what Aqpti posted, all I can say as make sure to give as much detail as you can so the real pros can help. Helps keep them from getting testy.