Improve README

README.md

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 The [TAP Core Schema](core-schema.adoc) defines a dictionary of common
 attributes used across various components of the TAP portfolio. The
-authoritative Core Schema is abstract -- it is not bound to any particular
-representation. However, to be useful the Core Schema must then be mapped to a
-specific representation, such as JSON.
+authoritative Core Schema is abstract -- it cannot be used directly, but is
+best thought of as an *attribute library* that can be used to construct other
+documents.
+
+The following types of documents are defined as part of the Core Schema library:
+
+* **Representations**: Mappings of the Core Schema to a specific form, such as
+  JSON, XML, or YAML.
+
+* **Profiles**: Selections of Core Schema attributes for a particular purpose.
+
+* **Protocols**: A specification for implementation, based on a Representation
+  and perhaps a Profile. Of all the Core Schema documents, only Protocols are
+  actually implementable in software.
+
+As an example, the **eduPerson LDAP Schema** would be a *Protocol* that
+references the *eduPerson Profile* and the *LDAP Representation*.