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Effectively they are both the same. A decoupling capacitor is used to decouple it from other parts of the circuit/sub-circuit like switching transients, voltage droops etc. It is also used to for providing transiet power for that particular circuit.
Other than this it also serves the role of a bypass(capacitor/network) which essentially shorts the high frequency/unwated signals in the DC power line.
Ok, I think that everyone has a good point on it, but it would be better if you do the math:
Start with the impedance of the capacitor. Its value is 1/ωC, wich means that it is invercly proportional to the frequency of a signal applied to it! There you are:
Decoupling capacitor of DC : It filters out all the zero frequncies and leaves your power line clean of "dirty" DC components.
Bypass capacitor, is a Decoupling capacitor from other frequencies: In wich part of the circuit are you going to use it? Which is the unwanted frequency components of the signal you want to get rid off? Do the math and you will have the value together with a resistor to act as a bandpass, lowpass or a high pass filter.
I hope it helped a bit ( I had the same problem before figuring that there is nothing more than a math expression to understand the physics behind this terms).