Encyclopedias provide fundamental information about a wide range of subjects, sometimes organized around specialized or discipline-specific content. For example, you may come across an encyclopedia of folk music that concerns itself exclusively with topics, vocabulary, movements, and key figures related to folk music, while a general work like the Encyclopedia Brittanica will provide information about a much larger range of topics, likely including folk music as a topic.
Encyclopedias are great for establishing basic historical or biographical information, and for getting acquainted with the kind of language used to talk about a particular subject.