Probably a no-brainer question here for you all

but I am writing a very simple chat (yes, another one...) application. Basically I just want it to be all 1-1 based via a
servlet on the server.
So a client initiates a conversation with another user and each response gets pushed into each others text area.
I understand the mechanics of it simply enough but am having trouble understanding how to push the response back from the servlet on the server to the
applet on the clients PC without them needing to refresh ?
As I said, probably very easy but any help in the right direction would be much appreciated.
Thanks everyone,
Jon