MPU/CPU generally refers to the main brain board of the game, usually in a metal try that slides in and connects to the motherboard/back plane.
a flash board is a circuit board with flash memory that stores a programmed game that attaches to a 044/4400 series mpu.
daughter boards (to me) are like the sound boards that attach to 3902 style mpus. there are eprom and sim style versions of the sound boards.
generally the easy way to tell the difference between 3902 and 044 is the type of handle on the tray, if it has a handle that lays flat and can lifted to pull out that would be a 044 board. if it has a fixed pull tab type it is generally a 3902/3802 style.
there is also a legacy adapter that gets attached to a 044 style board that allows you to install eprom/simm style games from the 3902 era.
hope this helps you a little bit.