Reference sources provide good background information on historical periods, art forms, styles, and movements (e.g. Goya, cubism), in addition to definitions of art terms and biographical information on artists. These sources are a great place to start your research.
The following websites provide collections of links to hundreds of individual websites relating to art and art history. Many of the resources collected on these sites have been vetted by academics, librarians, or others knowledgeable in the field of art.