What is the difference between "Work Item" and "Configuration Item"?

Some trackers are classified as “Work Item” (e.g. Requirement, Test Run, Issue, Epic, User Story), some as “Configuration Item” (e.g. Test Case, Test Set, Release, Configuration Items).

1. Is there a description what is meant with “Work Item” and what with “Configuration Item”?
2. What is technically the difference between a “Work Item” and “Configuration Item”?
3. How do I know if a tracker type is “Work Item” or “Configuration Item”?
4. Why belongs the tracker type “Requirement” to “Work Item” and not to “Configuration Item”?

BR,

Markus