though it is a very common quesiton but see pld stand for "programmable logic device" and CPLD for complex pld then FPGA for field progrramable gae array"see they are reprogrrammabe logical devices mace of some basic units may be a gate or mux or decoder depends upon its design.acutually they come in sequence of complexity required for the circuit u want to implement the col3st one is'pld' then cpld' then the last one