DICOM defines its own model to map relevant entities from the real world. That model, as defined in Part 3 of the standard, is shown in the figure below.
DICOM data model entities do not always map to DICOM objects! In fact, every DICOM object you will ever encounter in IDC will contain attributes describing various properties of the entries at different levels of the real world data model. Such objects are called Composite Information Objects. The information model of the Composite Information Object Definitions is shown below, and covers all of the composite objects defined by the standard.
As can be observed from this diagram, "each Composite Instance IOD [Entity-Relationship] Model requires that all Composite Instances that are part of a specific Study shall share the same context. That is, all Composite Instances within a specific Patient Study share the same Patient and Study information; all Composite Instances within the same Series share the same Series information; etc." (ref).
Each of the boxes in the diagram above corresponds to Information Entities (IEs), which in turn are composed from Information Modules. Information Modules group attributes that are related. As an example, Patient IE included in the MR object will include Patient Information Module, which in turn will include such attributes as PatientID, PatientName, and PatientSex.