Help a noob undrstand the XML architecture

I see a lot of different terms like:

• Tuple T
• Instance I
• Variable Tuple V
• Class C
• Name N
• ETC

And "stupid question from a noob" was wondering what each does? (I couldnt find an explanation that dosn't confuse me ).

Also about architecture 
I see a lot of XMls containing  structures like:
U inside V inside L inside I or L inside U  etc...
How is defined which tag can contain each?

Another point sometime I see parts of code containing the class and was wondering is the Class an obligatory Term?

Another thing:
 When I should use Name Attributes, Class Attributes or Tuple Attributes inside a structure?

Thank you for the patience in advance!