What is the difference between well formed XML and valid XML?

Related questions

1. What is XML Schema instance?
2. What is XML document structure?
3. What is the requirement of XML schema?
4. Why are XML files used?
5. What is XML schema file?
6. What is a valid document in XML?
7. Is XML well formed?
8. What is XML DTD explain?
9. What are the three main types of XML documents?
10. What language is used for defining XML schema?

Tags

1. #valid
2. #require
3. #constraints
4. #either
5. #document
6. #complies
7. #distinct
8. #validation
9. #schema
10. #order
11. #associated
12. #formed
13. #standard
14. #specified
15. #definition
16. #however