HealthUnity has a pretty good explanation of HIE technology including their HIE Use Cases. Documents on transmitted through HIE systems today typically conform to the HL7v3 Clinical Document Architecture (CDA). The Continuity of Care Document (CCD) is the most common of these, meant to be what your doctor needs to know at a glance. It looks something like this:
It's a clinical document with basic information about you and your doctors (any health care providers, really), information about the document itself (author, time of creation, purpose, etc), and a structured body which contains some number of sections - one for each category of health information being shown. For example, allergies, vaccinations, and current medications may be of immediate use, with the latest results, vital signs, and diagnoses. Much more can be documented depending on the purpose of the document.
To summarize: You (the patient) are the record "target." Every provider involved in the events documented is conveniently listed up front with contact information, followed by one or more sections. A section has a human-readable title and text followed by any number of coded entries. Each section is identified by its templateId (and so is the document itself, with its own templateId on the ClinicalDocument level).
Which sections are included depends on the document's purpose. A summary document has a bit of everything, while a lab or radiology report may have only the results section. There are many of these specified by HITSP (composite documents start with a "C" for example C32 is CCD and C84 is History & Physical).