It's a socket that supports the IEEE 1394 protocol, otherwise known as FireWire or i.Link. That' a very fast (generally faster than USB2) data/communications protocol that has a number of features, such as allowing computers to control certain aspects of attached camcorders, allowing chaining of peripherals, and sometimes providing power to peripherals (the 4-pin cables/ports don't provide power, but the others do).
There are two types of FireWire in common use, FireWire 400 and FireWire 800 (the numbers refer to speeds, in MB/sec). Pictures of the connectors can be found here:http://kb.wisc.edu/showroom/page.php?id=3113
Hope this helps!