1. A computing device having a processor and implementing a first schema for querying a first data store described by the first schema, the first schema being stored as a data structure on one or more computer-storage media accessible by the processor, the first schema comprising: a first property description element describing a first individual property of the data store including one or more static attributes and one or more contextual attributes that combine to describe the first individual property wherein each static attribute has a corresponding static attribute value and each contextual attribute has a corresponding contextual attribute value, wherein a static attribute associated with the first individual property is immutable and has the same static attribute value within all schemas that include the first individual property, wherein a contextual attribute is able to have different contextual attribute values among different schemas that include the first individual property, and wherein contextual attribute values are defined within each schema that has the contextual attribute; a property reference element that defines a second individual property in the first data store by referencing a second property description element from a second schema that includes static attributes and corresponding static attribute values wherein the property reference specifies contextual attributes and corresponding contextual attribute values, wherein the static attributes apply to all schemas that reference the second property description element, and wherein the contextual attributes apply only to the first schema; an item type description element describing an item type for at least one item in the data store, the item type description element including one or more properties that define the item type; and a kind description element describing a kind for at least one item in the data store, the kind description element including one or more properties for the kind, wherein the kind is a collection of logically related item types.