Netweaver Concept

Hai All,

I have been working past 6 month in sap basis,

but i couldn't understand the concept of netweaver and ERP.

I required some clarification, that is

1. What is the relationship between ERP and Netweaver

2. Netweaver '04 vs Netweaver '07

3. What is the Hierarchy in Netweaver.

4. Web application Server Vs Ecc (5,6)

any document is available to understand the concept, Kindly suggest

Thanks in advance

Sundar.C