What is a WEBFARM??

Question

What is a WEBFARM?? I DIDNT UNDERSTOOD IT FROM THE BOOK.. IS SAYS..."SIMILAR SERVERS"..but wat is meant by that? it says that .."In a webfarm scenario our request can get routed to another server and our session data is inaccessible if stored inprocess".. but why should our request get routed to other server..is our whole application not placed on one server??? what is actually happening in a web farm scenario???