Power/ground noise is usually transient in nature and caused by the currents drawn by devices when they switch. If the required current cannot be delivered to the switching devices, then the voltage along the power delivery network (PDN) is likely to drop. This voltage drop also happens for all other devices connected to that power rail. This is referred to as IR drop.
Power saving techniques, such as power gating, can exacerbate supply noise because they can cause large inrush currents.
Decoupling capacitors can be used to lessen power supply noise by providing small amounts of local charge that can be used to meet peak current demands. Power grid design is undertaken to lessen the impacts of this noise and to isolate sensitive circuitry. The package also plays a large role in the design of the power grid because large inductance can be created at this level which can also cause problems.
Analysis and mitigation of noise caused through the power delivery network is usually referred to as Power Integrity.