An FPGA is an integrated circuit containing a matrix of user-programmable logic cells, being able to implement complex digital circuitry. These cells are designed to implement user-defined combinational and sequential functions. FPGA families from different vendors use different logic cells architecture.
In this article we compare logic cells architectures that are used in modern FPGAs: Xilinx (both Virtex-5 and earlier), Altera and Actel. Here you can find a short overview of different FPGA logic cell architectures, as well as an attempt to elaborate a universal method of their comparison.