BP in CRM

Hello all

a newbie here.

I am on a clean installed system.

what are building blocks ?

and how are they different /or applicable to Best practises.

Please let me know in a layman language( without sap help links)

I would also lke to know what is the principal difference between the 'preconfigured scenarios' & 'building block library' on the best practices site.?

where should one start first , I am assuming the 'preconfigured scenarios', but then when is the need to come to the 'building block library'. Both seem to have the same information. also what is the 'Key Capability' coloum means in the bb library. lot of questions.

Thank you

Sharmishta