Manufactured IC types require at least a few weeks and often several months to convert a desired digital circuit design into a physical IC. What if a designer is developing a circuit that should be implemented today or the designer simply does not wish to incur the NRE cost, complexity and risk of manufacturing an IC types. An off-the-shelf IC is pre-manufactured and available for purchase from a store or more commonly by shipment from a vendor. A programmable IC is an off-the-shelf IC that implements a desired circuit simply by storing a particular sequence of bits into the IC's memory. Programmable ICs have the drawback of worse performance, sixe and power compared to custom or semicustom manufactured ICs. However, the implementation on a programmable IC can be ready in just minutes, with no NRE cost or other complexities of manufacturing an IC and these benefits may outweigh the drawbacks.

The most popular type of programmable IC is known as a field programmable gate array, or FPGA which consists of numerous configurable logic blocks and programmable interconnects, which shall be introduced shortly, that can be programmed to implement a desired circuit.

An FPGA company prefabricates an FPGA chip, meaning that the chip contains all the transistors and all wires that the chip will ever have, likely using a full custom IC type. A designer buys those FPGA chips, and then programs the chips to implement the desired circuit.

To program in this context means to download a series of bits into the chip's memories not to be confused with writing high level software programs like C or C++ code. Programming an FPGA occurs in the field meaning in a designer's lab or office or home as opposed to in a fabrication plant. Hence, the words field programmable in the name. Furthermore, programming typically takes only seconds or perhaps minutes at most.

The term gate array is there in the name because when FPGAs first became popular in the mid 1980s, they were marketed as an alternative to the gate array IC type, which was very popular at that time. Thus, an FPGA was a semicustom IC that could be programmed in the field instead of at a fabrication plant. However, be forewarned that the internal design of an FPGA chip looks nothing like a gate array the naming is somewhat unfortunate.

The two basic types of components inside an FPGA are lookup tables and switch matrices.