Oxford Art Online
If you are just beginning to explore a topic, an encyclopedic source like Oxford Art Online can be helpful. OAO includes essays on various topics, time periods, and movements in art history, definitions of art terms, and biographical information and exhibition history for individual artists.
Heilbrunn Timeline of Art History
The Metropolitan Museum of Art's Heilbrunn Timeline of Art History is another encyclopedic source. Search by artist name, object name, or keyword, or use the drop-down menus to narrow to a geographic area and time period.
From the Met's website:
The Metropolitan Museum of Art's Heilbrunn Timeline of Art History, originally launched in 2000, presents the Met's collection via a chronological, geographical, and thematic exploration of global art history. Targeted at students and scholars of art history, it is an invaluable reference, research, and teaching tool.
Authored by The Met's experts—curators, conservators, scientists, and educators—the Timeline comprises 300 timelines, more than 1,000 essays, more than 7,000 objects, and is regularly updated and enriched to provide new scholarship and insights on the collection.