Namespace or Assembly?

System.Data is a namespace, System.Data.DLL (the file) is an assembly.

A namespace is a logical grouping of types (mostly to avoid name collisions). An assembly can contain types in multiple namespaces (System.DLL contains a few...), and a single namespace can be spread across assemblies (e.g. System.Threading).

Namespace is a logical grouping of classes belongs to same functionality. So System.Web and System.Data are namespaces

MSDN describe it as:

Namespaces are heavily used in C# programming in two ways. First, the .NET Framework uses namespaces to organize its many classes Secondly, declaring your own namespaces can help control the scope of class and method names in larger programming projects.

Assembly is chunk of (precompiled) code that can be executed by the .NET runtime environment. It contains one or more than one Namespaces. A .NET program consists of one or more assemblies.

System.Web.dll and System.Data.dll are assemblies.

MSDN describe it as:

Assemblies are the building blocks of .NET Framework applications; they form the fundamental unit of deployment, version control, reuse, activation scoping, and security permissions. An assembly is a collection of types and resources that are built to work together and form a logical unit of functionality. An assembly provides the common language runtime with the information it needs to be aware of type implementations. To the runtime, a type does not exist outside the context of an assembly.

In short:

Assembly:

An assembly provides a fundamental unit of physical code grouping.It is an Output Unit. It is a unit of Deployment & a unit of versioning. Assemblies contain MSIL code.

Namespace:

A namespace provides a fundamental unit of logical code grouping.It is a Collection of names where in each name is Unique.They form the logical boundary for a Group of classes.Namespace must be specified in Project-Properties.