Acronym for Field Programmable Gate Array.
An FPGA is a (usually) digital device that allows in-system reconfiguration. In it's simplest form an FPGA is a bunch of functional units or CLBs/slices, which each have a multiplexor and a number of gates etc. These CLB's are connected by a matrix of interconnects with MOSFET to (dis)connect them. Although the exact layout of the chip is fixed, the electrical layout is re-configurable by re-programming the FPGA with a core. 
http://www.opencores.org has a whole bunch of cores (FPGA design files) written in VHDL or Verilog. You can include these in designs free of charge.
Xilinx are the biggest FPGA company at the moment, http://www.xilinx.com.
No page links to FPGA.
