Programmable Gate Array or FPGA as it is more widely called is a type of programmable device. Programmable devices are a class of general-purpose chips that can be configured for a wide variety of applications. These devices were constructed to implement logic circuits. The PLD included an array AND gates connect to an array of OR gates.
PALs come in both mask and field versions.
The PAL was designed for small logic circuits. A common MPGA consists of rows of transistors that can be interconnected to implement desired logic circuits. User sepcified connects are available both within the rows and between the rows. This enabled implementation of basic logic gates and the ability to interconnect the gates. As the metal layers ar defined at the manufacturer, significate time and cost are incured in producing the run.