What is an FPGA?
What does a logic cell do?
So what does 'Field Programmable' mean?
How are FPGA programs created?

What is an FPGA?

Before the advent of programmable logic, custom logic circuits were built at the board level using standard components, or at the gate level in expensive application-specific (custom) integrated circuits.  The FPGA is an integrated circuit that contains many (64 to over 10,000) identical logic cells that can be viewed as standard components.  Each logic cell can independently take on any one of  a limited set of personalities.  The individual cells are interconnected by a matrix of wires and programmable switches.  A user's design is implemented by specifying the simple logic function for each cell and selectively closing the switches in the interconnect matrix.  The array of logic cells and interconnect form a fabric of basic building blocks for logic circuits.  Complex designs are created by combining these basic blocks to create the desired circuit.

What does a logic cell do?

The logic cell architecture varies between different device families.  Generally speaking, each logic cell combines a few binary inputs (typically between 3 and 10) to one or two outputs according to a boolean logic function specified in the user program .  In most families, the user also has the option of registering the combinatorial output of the cell, so that clocked logic can be easily implemented.   The cell's combinatorial logic may be physically implemented as a small look-up table memory (LUT) or as a set of multiplexers and gates. LUT devices tend to be a bit more flexible and provide more inputs per cell than multiplexer cells at the expense of propagation delay.  

So what does 'Field Programmable' mean?

Field Programmable means that the FPGA's function is defined by a user's program rather than by the manufacturer of the device.  A typical integrated circuit performs a particular function defined at the time of manufacture.  In contrast, the FPGA's function is defined by a program written by someone other than the device manufacturer.  Depending on the particular device, the program is either  'burned' in  permanently or semi-permanently as part of a board assembly process, or is loaded from an external memory each time the device is powered up.  This user programmability gives the user access to complex integrated designs without the high engineering costs associated with application specific integrated circuits.

How are FPGA programs created?

Individually defining the many switch connections and cell logic functions would be a daunting task.  Fortunately, this task is handled by special software.  The software translates a user's schematic diagrams or textual hardware description language code then places and routes the translated design. Most of the software packages have hooks to allow the user to influence implementation, placement and routing to obtain better performance and utilization of the device.  Libraries of more complex function macros (eg. adders) further simplify the design process by providing common circuits that are already optimized for speed or area.