Assemblies question

I'm trying to understand the relationship between assemblies, namespaces and classes.  Can anyone help explain or point me to a usefule link?
1.  I understand that an assembly is typicall a dll or an exe (perhaps other files also).  Does each project compilation produce only 1 dll?  Or is the number of DLL dictated by something completely different?
2.  I read each assembly can contain multiple classes. Asingle assembly can contain classes for multiple namespaces, and asingle namespace can span multiple assemblies.  Can someone explain this?
Thank you!
John
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