What is the difference between sync and async web services?
from a webservice protocal perspective there is no difference. you still make a request and get a response. the difference is in what the initial response contains. in a synchronous call, the initial response will contain the final result. in an asynchronous call, the initial response will not contain the final result, but may contain some sort of "ticket" which can be used in a subsequent call to get the final result. generally, the client will need to poll the webservice until the final result is available.
Actually i am looking for a web service, which doesn't return a response. is this achievable?
it's not really possible to not return any response. http is a request/response protocol. if you don't have any data to return, then the response will be fairly minimal, but it will still exist.