I don't even have an ethernet shield yet I found your answer in less than two minutes. Screw google, until after you have exhausted all the standard issue sample code.
An http server returns information to a client, generally a web browser. However it requires a structured format. This page has an example of that. http://arduino.cc/en/Tutorial/BarometricPressureWebServer
Scroll down to listenForEthernetClients() There's tons more useful info there but that's the guts of it.