Grove Art Online is "the foremost scholarly art encyclopedia, updated regularly and covering global art and architecture from prehistory to present day. It includes peer-reviewed articles contributed by nearly 7,000 scholars from around the world, accompanied by images, bibliographies, and links to additional resources."
Reference books are helpful starting places if you need:
These general reference sources cover biographies, artistic movements and periods, and terminology.
The following are just a few of the reference sources that cover specialized topics.
Reference Sources for Modern/Contemporary Art