When you search an online encyclopedia, you are searching a compendium of information on just about everything. A compendium is a comprehensive collection of something.
You can also use the word compendium to describe a collection of written works. If you gather all of the anecdotes your parents and grandparents have told you into a book, you'll have created a compendium of family stories. The plural of compendium is either compendiums or compendia.