The 'visible web' is what one can find using general search engines such as Google, Ask, Yahoo etc. The 'invisible web' is what internet users cannot retrieve using search engines. The invisible web is also called as deep web, deepnet, or the hidden web.
Way back in the year 2000, it was estimated that the invisible web or the deep web contained approximately 7,500 terabytes of data and 550 billion individual documents. Estimates based on extrapolations from a study done at University of...