Freebase的数据存储格式叫Graph，nodes定义为/type/object，边被定义为/type/link.
Freebase文档中Topic的概念：topic就是entity，topic对应于图中的节点，但并不是所有的节点都是topic，还有Compound Value Types(组成似的的值类型，例如1997年温哥华的人口)
Freebase中的Topic有type，而且是一个topic对应多个type，而一个type又类似于一个模板或属性集合（文档中把type比作一个relational table）
type的集合叫做domain，每个domain都被分配了id，如：/business, /music等的第一级目录，而type的标识是第二级目录，如：/music.album，同样地，propert是第三级目标，如：/automotive/engine/horsepower
官方发summary：
- A type is a conceptual container of related properties commonly needed to describe a certain aspect of a topic.
- A topic can be assigned one or more types (the default type being
/common/topic) - As properties are grouped into types, types are grouped into domains.
- Domains, types, and properties are given IDs in a namespace/key hierarchy.
- Common well-known topics are given IDs in the
/ennamespace, which are human-readable English strings. - Topics are uniquely identified within Freebase by GUIDs.
- Properties are multi-value by default, and multi-value properties and single-value properties can be queried in the same way.