TDM vs. TDMA

TDM vs. TDMA:TDM stands for Time Division Multiplexing while TDMA stands for Time Division Multiple Access .The main difference between them is as follows : For both systems ,the user is given a certain time slot through which his data is to be transmitted then the user waits for the next frame to transmit at the same time slot .
For TDM,a certain time slot is always dedicated for a certain user even if the user isn't using it .An example of TDM is digital ground telephone networks (like ISDN) . 
For TDMA, after the user finishes using the time slot, the time slot is freed and can be used by another user .Usually time slots are dynamically assigned and the user may get a different time slot each time he accesses the network .An example of TDMA is GSM (actually GSM is a hybrid system containing both TDMA and FDMA) .