Examine the rules for resolving external references when linking separate modules. Modern systems are built around multifile inclusion, compilation, and linkage. For C++, it is necessary to understand how multifile programs are combined. Conceptually, files are individual modules of a larger program. Linking separate modules requires resolving external references.
The key rule is that external nonstatic variables must be defined in only one place.
Use of the keyword extern, together with an initializer, constitutes defining a variable. Using the keyword extern without an initializer constitutes a declaration but is not a definition.
If the keyword extern is omitted, the resulting declaration is a definition, with or without an initializer.

Consider these declarations in four files that would be linked:

char c;      //definition of c
.....

extern char c;    //declaration of c
.....

extern int n = 5; //definition of n
.....

char          c;  //illegal --
                  //second definition
extern float  n;  //illegal --
                  //type mismatch
extern int    k;  //illegal --
                  //no definition
.....

The purpose of namespaces is to localize the names of identifiers to avoid name collisions. The C++ programming environment has seen an explosion of variable, function, and class names. Prior to the invention of namespaces, all of these names competed for slots in the global namespace and many conflicts arose. For example, if your program defined a function called abs( ), it could (depending upon its parameter list) override the standard library function abs( ) because both names would be stored in the global namespace.
Name collisions were compounded when two or more third-party libraries were used by the same program. In this case, it was possible that a name defined by one library would conflict with the same name defined by the other library.
The situation can be particularly troublesome for class names. For example, if your program defines a class call ThreeDCircle and a library used by your program defines a class by the same name, a conflict will arise.

The creation of the namespace keyword was a response to these problems. Because it localizes the visibility of names declared within it, a namespace allows the same name to be used in different contexts without conflicts arising. Perhaps the most noticeable beneficiary of namespace is the C++ standard library. Prior to namespace, the entire C++ library was defined within the global namespace (which was, of course, the only namespace). Since the addition of namespace, the C++ library is now defined within its own namespace, called std, which reduces the chance of name collisions. You can also create your own namespaces within your program to localize the visibility of any names that you think may cause conflicts. This is especially important if you are creating class or function libraries.