Complex Programmable Logic Device (CPLD) is programmable logic device and can be programmed by using VHDL. CPLDs are based on EPROM or EEPROM technology. CPLDs are having extended density than the SPLDs. The concept of CPLDs is to have a few macrocells on a single chip with simple logic paths. CPLDs are classified depending upon the architecture which gives rise to high speed, detailed timing and simple software flow. The basic CPLD consists of configurable logic block (CLB) which consists of AND gate arrays and interconnects. The logic blocks are programmable AND, fixed OR devices. PALs and GALs are available only in small sizes, equivalent to a few hundred logic gates. CPLD is an arrangement of multiple SPLD-like blocks on a single chip. CPLD consists of multiple circuit blocks in chip. The circuit block in CPLD are same as that of PLA or PAL blocks. Figure below shows the example of a CPLD. This CPLD has four PAL blocks which are connected interconnection wires. The PAL block is also connected to a sub-circuit known as I/O block. The I/O block is connected to number of input and output pins. The PAL block consists of macrocells. The macrocell consists of flip-flop, a multiplexer, and a tri-state buffer. The flip-flop is used to store the output value produced by the OR gate. The tri-state buffer acts as a switch. In function block the AND array gets inputs from the I/O blocks and other function blocks. The product terms are given to fixed OR gates. The outputs of the multiplexor then sent through a clocked flip-flop. The function blocks are designed similar to PAL architectures. The I/O block is used to drive signals to the pins of the CPLD device. The CPLD interconnect is a programmable switch matrix. No switch can connect all internal function blocks to all other function blocks, therefore flexibility is achieved. 
Altera-FLASH-logic-CPLDs
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Altera-FLASH-logic-CPLDs
Advantages of CPLD :
1) Easy to design : CPLDs gives simple way to implement a designs.
2) Lower cost : CPLDs require low costs due to the feature of re-programmable.
3) Large product profit : CPLDs require very short development cycles because of which products time to market is faster and generates the profit.
4) Lower board area : CPLDs has high level of integration.
5) Simple design changes due to re-programming.
6) CPLDs are used in wide applications for prototyping small gate arrays