Namespaces are logical, whereas assemblies are physical.
A namespace is a logical naming scheme to group related types. Namespaces can contain other namespaces to form a hierarchy. The “fully qualified name” of a type is its namespace followed by its type name, separated by a period (for example, System.Windows.Forms.Button). Type names must be unique within a namespace, but the same type name can be used in different namespaces.
An assembly is a physical deployment scheme to group related types. An assembly can contain one or many namespaces. A namespace can exist in one or many assemblies.