What is a Grid? In definition it is nothing but some horizontal and vertical lines cutting each other. Something like

How is that related to grid computing? Not actually. But if you think that there is a computer node available at each intersection of horizontal and vertical line, we will get close.

What exactly is grid computing? Think of the fact there are many average computers connected to each other and can share their processing power, how fast it can deliver solution. It is form of implementation of distributed computing and parallel computing. It can help using many average sized computer and giving impression on a super computer processing power.

http://computer.howstuffworks.com/grid-computing.htm
http://www.gridcafe.org/grid-in-30-sec.html
http://en.wikipedia.org/wiki/Grid_computing