UI Manager and Skins

Can anyone provide a good explanation of:

1) What does UI mean?
2) What is the UIManager's purpose (it seems to manage html elements,
but a more detailed explanation would be great)
3) What are considered UI elements ?
3) How do skins work? ie. What is a skin? Are they loaded for each page?
What is the directory hierarchy for skins?

Thanks alot,



Chris