Relationship between the kernel and Netweaver?

Hi there

Could somebody explain the difference between the kernel and Netweaver?

I used to believe these were the same thing but I see the versions of the kernel and Netweaver can be upgraded separately and there are restrictions between the versions.

I am asking in the context of, say for example, ECC 6.0 ERP. What is the relationship between, for example, SAP Netweaver 7.5 and SAP Kernel 7.45?

Thanks in advance!

Joe