Basicaly bypass means to alow the passage of current around a component say to pass on higher frequencies in a tone network .
Decouple means to remove or seperate that component by shunting the current to ground usualy by means of a capacitor say in a powersupply.
bypass capacitor is used to provide a low impedance (in a special frequency) arund a device. such as the capcitor which used to bypass the emiter resistance. decoupling capcitor is used to separate the dc voltage and ac voltage. such as the capacitor between the output of one stage and input of the next stage. This causes the dc voltage of the previous stage separate from the ac voltage.
bypass is used when you want to ground a node in a desired frequency and that element only effect on the dc voltages and currents of your circuit.
decoupling is used to isolate two stage of a circuit and therefore therefore these two stage don't have any DC effect on each other.