Azzu, here are some links describing APPLY clause ; all the qustions you asked is explained in detail in the link ; i had given the link to the first part, the link to the second part is in the first part itself:

Link : Understanding and Using APPLY (Part 1) - Paul White[/url]

I tell you, this is by far the **BEST** article on APPLY i had ever read.. Thanks to Paul White for writing that up 🙂