Encyclopedias provide brief introductions to topics - in these cases topics related to Ancient Greece and Rome. Entries typically range from a few paragraphs to a few pages long.
Dictionaries tend to offer very brief definitions and descriptions of topics - in these cases topics related to Ancient Greece and Rome. Entries range from one to a few paragraphs long.
Handbooks, companions, and other types of reference works can also help you understand unfamiliar topics and find explanations of new concepts and terms.