The difference between Server.Transfer and Response.Redirect are as follow

1>Response.redirect sends message to the browser saying it to move to some different page while Server.transfer does not send any message to the browser but rather redirects the uyser directly from the server itself.So in Server.transfer their is no round trip while Response.redirect has a round trip hence puts a load on the server.

2>Using Sever.transfer you cannot redirect to a different server itself while using Response.redirect you can redirect to a different server also.

3>With Server.transfer you can preserve your information.It has a parameter called as "preserve form",so the existing query string etc will be avilable in the calling page.This is not possible in Response.redirect.