Point your mom at this entry from howstuffworks.com: http://computer.howstuffworks.com/grid-computing.htm.
At its most basic level, grid computing is a computer network in which each computer’s resources are shared with every other computer in the system. Processing power, memory and data storage are all community resources that authorized users can tap into and leverage for specific tasks. A grid computing system can be as simple as a collection of similar computers running on the same operating system or as complex as inter-networked systems comprised of every computer platform you can think of.
Plus pictures and links with more details. Tip o’ the chapeau to the HPC Watercooler.