What does it mean when it says URL not found?

Related questions

1. Why does it say webpage not available?
2. Can I use IP address instead of URL?
3. What is a 303 status code?
4. What does error code 303 mean?
5. Why does 403 Forbidden error occur?
6. What causes 403 Forbidden error?
7. Why would I get a 403 Forbidden error?
8. What HTTP status code 401 indicates internal server error?
9. What does HTTP Error 303 mean?

Tags

1. #accordingly
2. #webpage
3. #client
4. #either
5. #typed
6. #trying
7. #changed
8. #could
9. #which
10. #error
11. #reach
12. #incorrectly
13. #found
14. #removed
15. #means
16. #server
17. #moved