Grounding
"Every conductor has a finite resistance... If one point is defined as a zero of potential, then the assumption that a nearby point is also at zero will usually be incorrect."
--- Ralph Morrison, Grounding and Shielding Techniques in Instrumentation
Safety First Grounding is an essential component of electrical safety in applications ranging from AC wiring in buildings to consumer products. In these safety applications there is ultimately a physical connection to the Earth.
Common Grounds In other applications, ground and grounding simply refer to a common connection that may or may not connect to earth. In an automobile, the frame or unibody is often used as a common connection and called ground. In electronic equipment there may be multiple different common connections each with their own name & schematic symbol. For example: chassis ground, signal ground, analog ground, digital ground and power ground, to name a few. How and where these might be connected together can be a complex and controversial subject.
Grounding Schemes Single-point or star grounding attempts to control noise and common impedance coupling by routing separate signals in separate conductors to one point. This can work at audio frequencies but is impractical for RF and digital circuits. Multi-point grounding is required for controlling radiated noise and immunity in RF and digital circuits.
Grounding in the Real World In a typical desktop computer, the metal enclosure is connected to the motherboard ground plane, which is connected to the AC equipment safety ground. The first reason for this is electrical safety. The second reason is for reducing electromagnetic emissions and improving radio frequency immunity. Unfortunately the AC power ground is not at "zero of potential" and is likely to be a conduit of noise from multiple sources. These sources include other equipment in the building and external sources, like radio and TV transmitters.