The Age of Sand concept came about in a speech Douglas Adams gave at the Digital Biota 2, in Cambridge."Is there an artificial god" was a discussion about God and the impact the concept had on humanity (this speech is featured in The Salmon of Doubt).
The Ages of Sand is Adam's description of the technological changes humanity went through and how those changes broadened our understanding.
The first age is that of astronomy, when we looked at the stars and realised our belief that the Earth is the center of the universe was false.
The second age is of the microscopic, when we looked down at the tiny formations that make life and the universe and found something quite startling.
The third age is that of computers- the silicon chip which gave us process. The computer can perform mathematics very fast and this allows us to create models, simulations. This gave us another form of perspective as where can see how life works with these models and simulations.
The fourth age of sand is the one we are entering, is that of the Internet. This runs on fibre-optics and is the fourth form of communication many-to-many (the other three being, one-to-one (telephone for example), one-to-many (broadcasting, journalism, etc) and many-to-one (democracy is a model of this).
This speech can be found here: http://www.biota.org/people/douglasadams/