Each tag is a backtick-quoted string with key:"value" pairs. The json key controls how encoding/json sees the field: a plain name renames the JSON key, - excludes the field entirely, and ,omitempty causes the field to be omitted when its value is the type's zero value (empty string, 0, nil pointer). Always tag fields explicitly even when the JSON key matches the Go name; the explicit tag survives renames and signals intent. Note that struct fields must be exported (capitalised) to be marshallable at all.

Nested struct fields produce nested JSON objects automatically; their inner tags control the nested keys. If you embed a struct anonymously (Address with no field name), Go's JSON marshaller flattens its fields into the outer object, sometimes a useful shortcut and sometimes a footgun when you want explicit nesting. Pointer fields marshall as null when nil, which combined with omitempty is a clean way to express optional sub-objects. For maps and slices, the JSON output is automatic; tag the outer field to control the key, the inner element types are encoded with their own tags.

Pass a pointer to a struct; the marshaller fills the fields whose tag (or name, if untagged) matches a JSON key. Type mismatches return an error; missing fields stay at their zero value. By default, unknown JSON keys are silently dropped, which is helpful for forward-compatible APIs but dangerous when you genuinely want to detect typos. Use json.NewDecoder(r).DisallowUnknownFields() for strict mode. Use json.RawMessage as a field type to defer parsing of a sub-object you do not want to interpret yet.