Is window really global in Javascript?

The reason why you can access "out of scope" or "free" variables in ECMAscript is the such called Scope chain. The scope chain is a special property from each Execution context. As mentioned several times before, a context object looks at least like:

• [[scope]]
• Variable / Activation Object
• "this" context value

each time you access a variable(-name) within a context (a function for instance), the lookup process always starts in it's own Activation Object. All formal parameters, function declarations and locally defined variables (var) are stored in that special object. If the variablename was not found in that object, the search goes into the [[Scope]]-chain. Each time a function(-context) is initialized, it'll copy all parent context variable/activation objects into its internal [[Scope]] property. That is what we call, a lexical scope. That is the reason why Closures work in ECMAscript. Since the Global context also has an Variable Object (more precisely, **the variable object for the global object is the global object itself) it also gets copied into the functions [[Scope]] property.

That is the reason why you can access window from within any function :-)

The above explanation has one important conceptional conclusion: Any function in ECMAscript is a Closure, which is true. Since every function will at least copy the global context VO in its [[Scope]] property.

Is window really global in Javascript?

Yes. Unless you create a new variable called window in a narrower scope

function foo() {
    var window;
}

Inside foo we can access window, we all know that, but why exactly?

Any function can access variables declared in a wider scope. There is nothing special about window there.