SFP is a Gigabit Ethernet transceiver that takes up just a little more physical space than one RJ-45 port. The optical ones use LC connectors.
GBIC is a Gigabit Ethernet transceiver that takes up about 2 to 3 times the amount of physical space of an RJ-45 port. The optical ones use SC connectors.
GBICs were first in Cisco's product line; SFPs came later. You can fit more SFPs in a given amount of space than you can GBICs, which is why you're seeing more and more products come with SFPs.
While they can be interconnected with the correct cables, they are not interchangeable because of the significant difference in size. A GBIC can't fit in an SFP slot, and an SFP will not work in a GBIC slot.
Physical size, and the optical connectors they use, are the only real differences between them. Price-wise, they are identical: list price on a Cisco SFP is the same as the list price of the equivalent Cisco GBIC.
Hope this helps.
