I don't understand the difference between the two in relation to the subject at hand to say, I've seen both terms used in what I've been reading so far - but yes, I probably should have said ASIC vs. FPGA. I just had to Google the difference so I'll chalk that up to learning something new today
Basically, the one thing an ASIC was programmed to do will be the only thing it will ever do in all its life.
An FPGA, on the other hand, is a reprogrammable chip. But it doesn't run an OS. It's freaking fast because it's all hardware - no software lagging and no possibility of a crash. You flash a bitfile ("program" which tells the FPGA what operations and calculations it will perform internally) you power it up, and off it goes!
Now, I'm unaware of the use of FPGAs in the crypto mining world. Probably the bitfiles that do the hashing algorithms aren't publicly available. If they would be, you could theoretically use an FPGA to hash any algorithm, just change the bitfile.
Edit: FPGAs are usually used by developers who want an inexpensive way to test their algorithms before commiting to an ASIC design. One they got all the nuts and bolts down by compiling and flashing several bitfiles in the FPGA, they can take that algorithm and put it into production using ASICs, which are cheaper to manufacture, but can't be repurposed like FPGAs do.