Volunteer computing

Volunteer computing is type of distribute computing in which normal users volunteer to contribute their computing resources which are not in use by them for projects which require it. This is similar to grid computing but experts always asks to distinguish between grid and volunteer computing. In volunteer computing the users may be anonymous which is not generally possible for a grid contributer. But desktop grid computing do have close similarity to volunteer computing. Both are inseparably related to normal PC users across the globe.
Also, a non desktop grid generally refers to sharing of resources within and between organizations in which each of them is 'responsible' for the maintainance and smooth working of the grid. Also each organisation involved can act as the producer or consumer.

Volunteer computing on Berkeley wiki
Volunteer computing in Wikipedia

Loose coupling

In computer science, coupling is the degree of interdependency between resources which is usually between software modules. Loosely coupled systems are the systems which depend and communicate very less each other. They do not make assumptions about resources involved in other units which provides a kind of independence. Therefore it does not become a necessity that one be changed when the structure of other units involved in the system are changed.

The idea of loose coupling was introduced by Karl Weick in organizational studies. It is used in grid computing.

Load baloancing

Load balancing is a technique used to distribute the load(work to be performed) evenly or unevenly between resource like computers, network links,CPUs or any other computer resources to get a better resource utilization and increased load handling capability. Since number of resources involved is more than one, it increases reliability because if any single unit involved fails, other can take over.

A very common example of load balancing system is a server farm to handle traffic.

What is Grid computing?

Grid computing is a method of doing computing tasks that demand high processing ability by splitting the task and dividing it into a network of computers as small ones such that each of them can individually handle. Looking from the other end, a grid may  be defined as a virtual computer with super computing ability made up of a network of computers( a cluster in fact) which acts together to form a highly efficient system able to process big tasks.