Hello vijay,
Web service we don't have hosting facility at many level or we do not have that much contract facility. as well WCF have these all functionality and it is more easy to use than WebService as well as for Silverlight application u must have to go for WCF Functionality.
WCF is support many protocol ( like HTTP,TCP/IP,PIPE,MSMQ).Its give a more security mechanism(Like Transport,Message,TransportWithMessageCredential).We can be hosted it outside the IIS i.e Managed Windows Application,a Windows Service,WAS(Windows Process Activation Service).WCF service provide customization such as event hook int service start/end/init/custom global error handling