FPGAs, love the damn things. They're circuits that you can re-program at will after they've been manufactured! If you build, like, a 2-input AND gate, that's all it will ever be. It can only take in 2 inputs and AND them together. But with an FPGA, they're manufactured to be versatile; you "program" the circuit you want to achieve onto the chip, and it will achieve that functionality! You can make a 2-input AND gate, slap it onto a bread board, and have yourself that nifty little AND gate, but if you later decide you wanted it to be a NAND gate, just reprogram the chip and like magic, what was once an AND gate is now a NAND gate. They're great!