Added overview section [skip ci]

docs/guide/README.md

@@ -30,6 +30,7 @@ Getting Started
 Application Structure
 ---------------------
 
+* [Overview](structure-overview.md)
 * [Entry Scripts](structure-entry-scripts.md)
 * [Applications](structure-applications.md)
 * [Application Components](structure-application-components.md)

docs/guide/structure-overview.md

+Overview
+========
+
+Yii applications are organized according to the [model-view-controller (MVC)](http://wikipedia.org/wiki/Model-view-controller)
+design pattern. [Models](structure-models.md) represent data, business logic and rules; [views](structure-views.md)
+are output representation of models; and [controllers](structure-controllers.md) take input and convert
+it to commands for [models](structure-models.md) and [views](structure-views.md).
+
+Besides MVC, Yii applications also have the following entities:
+
+* [entry scripts](structure-entry-scripts.md): they are PHP scripts that are directly accessible by end users.
+  They are responsible for starting a request handling cycle.
+* [applications](structure-applications.md): they are globally accessible objects that manage application components
+  and coordinate them to fulfill requests.
+* [application components](structure-application-components.md): they are objects registered with applications and
+  provide various services for fulfilling requests.
+* [modules](structure-modules.md): they are self-contained packages that contain complete MVC by themselves.
+  An application can be organized in terms of multiple modules.
+* [filters](structure-filters.md): they represent code that need to be invoked before and after the actual
+  handling of each request by controllers.
+* [widgets](structure-widgets.md): they are objects that can be embedded in [views](structure-views.md). They
+  may contain controller logic and can be reused in different views.
+
+The following diagram shows the static structure of an application:
+
+![Static Structure of Application](images/application-structure.png)