the term 'deep web' refers to the subset of internet-connected information that is not widely published eg on search engines, where as the 'dark web' is specifically sites that hide their hosting information behind tor i2p etc

as unfair as it may be, a huge part of the usefulness of information is its accessibility, and these search engines currently hold a near-monopoly on which sites can generally be considered readily accessible, ie the 'surface web' above the deep web