First of all, FPGA's are NOT configurable microcontrollers, although many modern devices have processors built in. At the most basic level, they are substitutes for glue logic, or what we used to build digital logic with, 4000-series, 74-series IC's, etc. In other words, where you would previously buy a bunch of chips with logic, flip-flops, buffers... and then wire them together to create your system, now you simply program your logic design into one of these marvelous devices, and just wire it onto your board, and that's your project. Further, you can buy FPGA's with embedded devices such as processors, memory, DSP, etc.
The subject of FPGA could easily fill a thick book ( and it does ) Hope my ten cent explanation helps.