Complex Programmable Logic Devices (CPLDs) are integrated circuits that offer customizable logic functionality. They evolved from simpler PAL and GAL devices, featuring larger scale and complexity, belonging to the category of high-density ICs. 

CPLDs allow users to design and implement digital logic circuits according to specific requirements, combining programmable logic blocks with interconnect resources. These programmable logic devices facilitate rapid prototyping and flexible design in various applications, including interface logic, control systems, and signal processing, offering a middle ground between basic PLDs and more complex FPGAs.