Faster casting rate is found on a variety of equipment. This stat is most important for spell-casters, for obvious reasons. All spells are affected by this value, but note that some spells have a 1 second, or longer, delay between uses. For those spells, FCR equipment will lower the frames required for the actual casting animation, but the 1 second delay will still run (for 25 frames) after each cast.

For instance, if your character takes 11 frames to cast a spell that has a 1 second delay, that's 11 + 25 = 36 frames per use. If you you use FCR equipment to cut your casting frames to 7, you'll still have to wait 7 + 25 = 32 frames to cast it again. Saving 4 frames ever time you cast a spell will certainly add up over hundreds of casts, but the difference is less noticeable than it is with spells you can "spam" as quickly as your character can cast them.