Dictionary: Provides the meaning of words in a language or field of knowledge.
Encyclopedia: A single or multi-volume reference work that provides general background articles on either a wide range of topics or a more specialized discipline.
Handbook: A compact reference book giving the essential information in a given field of study.
So what's the difference...
The following tools from Medline Plus may be useful.
Health Topics--overviews of body systems, disorders, diagnosis, & more.
Medical Encyclopedia--easy-to-read articles about diseases, symptoms, & more.
Understanding Medical Words--to help you understand medical lingo.
NIH MedlinePlus magazine--"We bring you the latest breakthroughs from NIH-supported research. We feature people from all walks of life talking about how they've handled their health challenges."