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A decoupling capacitor is a capacitor used to decouple one part of an electrical network (circuit) from another. Noise caused by other circuit elements is shunted through the capacitor, reducing the effect they have on the rest of the circuit.An alternative name is bypass capacitor as it is used to bypass the power supply or other high impedance component of a circuit.
One common kind of decoupling is of a powered circuit from signals in the power supply. Sometimes, for various reasons, a power supply supplies an AC signal superimposed on the DC power line. Such a signal is often undesirable in the powered circuit. A decoupling capacitor can prevent the powered circuit from seeing that signal, thus decoupling it from that aspect of the power supply circuit.
Another kind of decoupling is stopping a portion of a circuit from being affected by switching that happens in another portion. Switching in subcircuit A may cause fluctuations in the power supply or other electrical lines, but you do not want subcircuit B, which has nothing to do with that switching, to be affected. A decoupling capacitor can decouple subcircuits A and B so that B doesn't see any effects of the switching.
To decouple a subcircuit from AC signals or voltage spikes on a power supply or other line, a bypass capacitor is often used. A bypass capacitor is to shunt energy from those signals or transients past the subcircuit to be decoupled, right to the return path. For a power supply line, a bypass capacitor from the supply voltage line to the power supply return (neutral) would be used.
High frequencies and transient currents flow through a capacitor, in this case in preference to the harder path through the decoupled circuit, but DC cannot go through the capacitor, so continues on to the decoupled circuit.


Switching subcircuits
In a switching subcircuit switching noise must be suppressed. When a load is suddenly applied to a voltage source, the circuit tries to suddenly increase its current , but the inductance in the power supply line acts to oppose that increase. It opposes it by lowering the voltage the power line supplies. This is not just the voltage that the load in question sees, but the voltage that every other subcircuit that shares that power supply line sees. This is only temporary -- the inductance ultimately loses the battle and the voltage comes back to normal. But even a temporary reduction in voltage can disturb other subcircuits.
To decouple other subcircuits from the effect of the sudden current demand, a decoupling capacitor can be placed between the supply voltage line and its reference (ground) next to the switched load. While the load is switched out, the capacitor charges up to full power supply voltage and otherwise does nothing. When the load is applied, the capacitor initially supplies demanded current. By the time the capacitor runs out of charge, the power supply line inductance cannot maintain the previous current, so the load can draw full current at normal voltage from the power supply (and the capacitor can recharge too). The voltage dip is reduced but not eliminated; i.e. the decoupling is not perfect.


Transient load decoupling
Transient load decoupling as described above is needed when there is a large load that gets switched quickly. The parasitic inductance in every (decoupling) capacitor may limit the suitable capacity and influence appropriate type if switching occurs very fast.
Logic circuits tend to do sudden switching (an ideal logic circuit would switch from low voltage to high voltage instantaneously, with no middle voltage ever observable). So logic circuit boards often have a decoupling capacitor between power supply and ground right next to each logic IC. These capacitors decouple every IC from every other IC in terms of supply voltage dips.

Placement
A transient load decoupling capacitor should usually be placed as close as possible to the device requiring the decoupled signal. The goal is to minimize the amount of line inductance and series resistance between the decoupling capacitor and that device, and the longer the conductor between the capacitor and the device, the more inductance there is.[1].
A power supply decoupling bypass capacitor should be placed as close to the voltage/current source as possible. The idea is to minimize the line inductance and series resistance between the capacitor and the supplied devices.
The guidelines for placing a high-speed decoupling capacitor on a multi-layer printed circuit board depend on whether the board has dedicated power distribution planes and how closely spaced those planes are. [2]