What is Freebase?
Freebase contains tens of millions of topics, thousands of types, and tens of thousands of properties. By comparison, English Wikipedia has over 4 million articles. Each of the topics in Freebase is linked to other related topics and annotated with important properties like movie genres and people's dates of birth. There are over a billion such facts or relations that make up the graph and they're all available for free through the APIs or to download from our weekly data dumps.
See Google Developers Guide for Freebase