Acronym for GigaBit Interface Converters
Used throughout Cisco's Product range to provide versitility amoungst their Gigabit uplinks.
Cisco offers a range of Gigabit Interface Converters (GBICs) and Small Form-factor Pluggable (SFP) for Gigabit Ethernet use. These small, modular optical interface transceivers offer a convenient and cost effective solution for the adoption of Gigabit Ethernet in data center, campus, metropolitan area access and ring networks, and storage area networks.
Have a look at Cisco's GBIC Range
There are two physical layers for running Gigabit Ethernet, you can run it over Cat5e (which is cheap), or you can run it over fiber (which can be a lot longer, is electrically inert, etc). However because fiber is so expensive, if you brought a switch with lots of fiber ports and wanted to use cat5e you'll end up shelling out a lot of money you don't need to. So instead Cisco made the ethernet physical layer a pluggable component called a "GBIC", so you can choose which type of fiber you want to put in any individual port. This idea was last seen as AUI when people were doing exactly the same thing, for the same reasons with 10mbit ethernet, except it was Coax/ThinNet/ThickNet.
Also Look at SFP