Earth vs Ground

see this

http://cr4.globalspec.com/comment/30343

Ground is a term that is regularly used to refer to a circuit common node. To make a positive power supply with reference to common (or ground) we tie the negative side of a power supply to the common node. We do the opposite to make a negative supply.

Earth is the planet we live on. It is also often used as a common node to make sure that electrical devices do not end up charging to many volts above earth ground. Static shock is an example of a relatively small charge difference between you and earth ground (or two isolated objects) being discharged. Without earth grounding many devices would develop dangerous charges. Earth ground is often tied to the chassis of electronic devices as well as the common electrical node.