What is difference between XML and valid xml?

You can compare this with English usage. I can say to you, "Jeet yet?" and you would probably understand me to mean, "Did you eat yet?" One of these sentences is valid English, the other is questionable.

In the same way you can write valid XML and you can write XML with numerous imperfections. Many XML language processors are capable of understanding both. However, it's safer to write valid XML.