whats the difference between url and destination using httpservice in flex?

I wondering whats the difference between url and destination using httpservice in flex.

I came across a httpservice example that looked something like this

        service.resultFormat="e4x"
        service.method="GET";
        service.destination="http://www.blash.com" // I used an actual page 
        service开发者_StackOverflow中文版.addEventListener("result", httpResult);
        service.addEventListener("fault", httpFault);
        service.send(parameters);

When I tried this I got a fault error saying that the services "channel" was null.

I took out the destination parameter and set the url parameter instead and it worked fine.

What exactly are destinations and channels all about?

FYI The page I'm calling is a public API that returns results in XML format.

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destination is a proxy which you have to configure in service-config.xml it's more related to remoting.