(Reference )

Reference is a section for sites devoted to the study of and access to information itself - teaching it ( Education ), using it ( Knowledge Management ), storing it ( Libraries, Museums ) - or sites containing information on topics varied and comprehensive enough that they could not be classified under the other top level The Directory categories ( Dictionaries, Directories, Encyclopedias ). Reference.

References can take on many forms, including: a thought, a sensory perception that is audible ( onomatopoeia ), visual (text), olfactory, or tactile, emotional state, relationship with other, spacetime coordinate, symbolic or alpha-numeric, a physical object or an energy projection. (wikipedia)