What is it called when a block returns a value?

It is not a namespace, it is a macro which returns maximum of two values.
\ at the end of the statements is use to append multiple statements and create a multi-line macro.

The code is not standard C++ but it compiles in gcc because it is supported as an gcc compiler extension.

Good Read:

Statement Expressions:
A compound statement is a sequence of statements enclosed by braces. In GNU C, a compound statement inside parentheses may appear as an expression in what is called a Statement expression.

         .--------------.
         V              |
>>-(--{----statement--;-+--}--)--------------------------------><

The value of a statement expression is the value of the last simple expression to appear in the entire construct. If the last statement is not an expression, then the construct is of type void and has no value.

Note: This excerpt is taken from IBM XL C/C++ v7.0 documentation.

This a macro, just like any other #DEFINE. Essentially, the compiler replaces MAX(a,b) with the code defined therein. This will return the max value.

This is called a statement expression, and is a non-standard extension of GCC. It allows you to use a compound statement as an expression, with a value given by the last expression in the compound statement.

It's used here to avoid the problem that function-like macros may evaluate their arguments multiple times, giving unexpected behaviour if those evaluations have side-effects. The macro is carefully written to evaluate a and b exactly once.

In C++, you should never need to do anything like this - use function templates instead:

template <typename T> T max(T const & a, T const & b) {
    return a > b ? a : b;
}

First of all, it is not Standard C++, because typeof is an extension to C++, by GCC. There is another extension, called Statement Extension is used in the code.

Compile your code with -pedantic option, it will not compile.

As for the question, it is not namespace. It is just a macro, which gives you maximum of two values.