From an optical perspective, the SFP modules and GBICs are essentially identical. SX SFPs talk to SX GBICs; LX/LH SFPs talk to LX/LH GBICs; etc.
From a physical perspective, they are different in size, so you can't substitute an SFP for a GBIC in the same switch. Some switches, like the Cat2950, Cat3500, Cat3500 series, use GBICs. Other switches, like the Cat3750, use SFPs. So you need to get the correct optical module(s) for the switch(es) you're using.
Because the sizes of these optical transceivers are different, the optical connectors they use are also different. The SFP uses LC connectors, which are like SC connectors, but smaller; while the optical GBIC uses SC connectors. So, this is an issue to pay attention to when you're buying fiber optic patch cables: you need to be sure to order ones with the right connectors for the SFPs or GBICs that you are using. And of course, the other ends of those patches have to be compatible with the fiber terminations you're connecting to (ST, SC, FC, LC, MT-RJ, etc.)
Cisco's SFPs are the same cost as their GBIC counterparts, last time I checked. Because they are smaller than GBICs, in theory you can fit more of them into the same amount of space. But unlike MT-RJ connectors, which are also designed to fit more ports into a limited amount of space, the SFPs with their LC connectors are modular rather than fixed. So you can mix and match to get exactly the types of connections you need (SX, LX/LH, ZX, etc.)
If you're familiar with GBICs, then it helps to just think of SFPs as mini-GBICs.
They (SFPs) are newer than GBICs, so there aren't as many devices in Cisco's product line that use them vs. GBICs. But since they are smaller, you can expect to see more of them in Cisco's products in the coming years. Other manufacturers are using them, too.
