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What is a StructureDefinition?

A StructureDefinition is the FHIR resource that defines the structure of other resources, data types, extensions, and profiles. It is both a resource (can be stored on a FHIR server) and a schema (defines what other resources look like).

Key fields

url — globally unique canonical identifier for this definition
kind — resource, complex-type, primitive-type, or logical
type — the FHIR type being defined or constrained (e.g. Patient)
baseDefinition — the parent StructureDefinition this one derives from
derivation — specialization (new type) or constraint (profile on existing type)
differential — only the elements that differ from the base — what you write
snapshot — complete element list after merging with base — generated by tooling

What uses StructureDefinition

Resource definitions — every FHIR resource (Patient, Observation…) is defined by a StructureDefinition in the spec
Profiles — constraints on base resources for specific use cases
Extensions — definitions of new data elements not in the base spec
Data types — HumanName, Address, CodeableConcept, Quantity