It's a book of several volumes, giving information on many subjects or on many aspects of one subject is known encyclopedia. An encyclopedia or encyclopaedia is a book or series of books that contain general information about many topics or areas. The word encyclopedia comes from the Greek word “enkyklia paideia”, which means "a general knowledge." The word has been in use for at least 500 years, and used in print for the first time in Encyclopaedia, or Knowledge of the World of Disciplines, which was published in 1559. An encyclopedia is often confused with a dictionary, although they are inherently different. A dictionary offers definition of words, while an encyclopedia explores topics more in depth and often includes illustrations, maps, and photographs. While most encyclopedias are organized alphabetically, some are organized by categories or cross-referenced articles. Many encyclopedias are now being published in CD-ROM form; other encyclopedias are now entirely available online, with no print version. One of the most important advantages of online encyclopedias is the fact that they can be edited frequently, so they remain up to date.