Http Tunneling

Hi There,

While going through technical jargons, I came across this term
'Http-Tunneling'. It is said that using this,it is possible to 
send data of any protocol through proxy over HTTP.Can any one tell me how
this
can be achieved? any sample code will provide good idea or send me the urls
where 
I can get the basics of http-tunneling.

Is it possible to tunnel complex protocols which are used by 'Microsoft
Netmeeting',
or AIM etc?

Thanks in advance!!!

yog