A round trip is the event generated onto a control which 
goes back to a server and executes the required process.On 
page load it goes back to the sever if the autopostback 
event is made true...PostBack is the event going to the 
server and coming back (in simple words)

round trip means the form ggoes to the server and take the 
response from the server and it may redirect to the other 
form or same form according to the request
where as postback means the form goes to the server and 
loads the same page