An issue is the document, bug, request, annotation, discussion item etc.
An issues can have events associated with it to signify issue state transitions,
poriority changes, owner assignment, etc. that either have occured or are
planned to occur some time in the future.
Links are used to relate issues with each other and with any other document
on the Web
Contacts can create issues and they can also resolve or track issues. The
issue creator and owner are two default relationships between
contacts and issues - other relationships can be registered as any other
relationship as the identifier of a contact is a URI (often an email address
so that we can handle email based notifications).
Skills (or interests)
Skills are used as the filter for figuring out when contacts should be notified
about changes in the database. Contacts can "listen" to all issues in a forum
of any particular category, priority, etc. but can also listen to a single
issue (for example the one they have created).