What are Etags used for?

Related questions

1. What is s3 ETag?
2. What is an ETag and how does it work?

Tags

1. #tagged
2. #cache
3. #validation
4. #entity
5. #resources
6. #header
7. #requests
8. #persistent
9. #browser
10. #etags
11. #identification
12. #elements
13. #browsers
14. #conditional