General Question: What is "SAP Application" and What is "SAP Component"?

I'm having a hard time understanding the context of SAP Application and SAP Component. When we say SAP ERP6 it is clearly that it is an application since we can install it as a "server". But when we say SAP SCM it can be a component or SAP ERP6 and we can install a server of SAP SCM. In this case it can be both?

SAP Netweaver Application is a application but SAP Netweaver Portal is an component? This confuses me.

How to differentiate between an "SAP Application" and "SAP Component"? Can you give me a couple of examples. Thanks in advance. Please correct me if I'm wrong

regards,

MJ