Silverlight or WPF web app

Hi There,

As*I a .NET person and just like started to build my new (Rich and secured) Internet application ,, I love to start with WPF or its child Silverlight as I'm learning lots about them... but, at the moment it got me too confuse to start as I'm not sure to build WPF web app or Silverlight web app!! or which one is the best environment to start, any differences, richer, secured*and future implementation features!!
*
tried to Google it but unfortunately, didn't get me the right path to the right answer!

why Microsoft published SL if you can build your web app with WPF?

Thanks