Need to refresh your memory about motifs in Chinese ceramics or the elements of Palladianism? The V & A style guides may have what you are searching for. Actually the museum offers a wide range of online databases, but the well-illustrated Understanding Styles section is particularly helpful when only a quick introduction is needed. Topics range from major historical periods in the decorative arts to the influence of Japanese design on the West.
Oh, and by the way, there is a quiz at the end of each sub-section.