What is mDNSResponder for?

[David Jarvie <lists@astrojar.org.uk>]

Evey time I boot up (Sarge), I get a list of messages dropped by my firewall. 
They are all for IP address 224.0.0.251, on UDP port 5353. It looks like 
these transmissions are being generated by mDNSResponder. I can't find 
anything more than a very cryptic description of what this program is for, 
but it looks as if it might be designed to run on server machines rather than 
desktops. Can anybody explain its purpose, so that I can decide if I actually 
need it?

Cheers,
David.