API for providing services

I have seen websites offering API's like this

script = "<script src='http://xxx.yyy.com/Widget.js?ID=" + widgetId + "&view=widget' type='text/javascript' ></script>";

Or

<iframe src="http://www.xyz.com/abcframe/index.php?ref=VD2000010&iframe=true&width=225&height=214&bg=ffffff&site=myname&from=AU,NZ&to=AU,NZ" frameborder="0" height="214" width="225" scrolling="no" ></iframe>

these api's are returning an Iframe or div'c containing contents from their website where the customer can use their services like book a room in a hotel or search for a flight from within another site. What would be the best way to provide this service from an asp.net site? If anyone have any reference link, please help me with it. Thank you.