guide WIP [skip ci]

docs/guide/structure-widgets.md

+Widgets
+=======
+
+> Note: This section is under development.
+
+Widgets are self-contained building blocks for your views, a way to combine complex logic, display, and functionality into a single component. A widget:
+
+* May contain advanced PHP programming
+* Is typically configurable
+* Is often provided data to be displayed
+* Returns HTML to be shown within the context of the view
+
+There are a good number of widgets bundled with Yii, such as [active form](form.md),
+breadcrumbs, menu, and [wrappers around bootstrap component framework](bootstrap-widgets.md). Additionally there are
+extensions that provide more widgets, such as the official widget for [jQueryUI](http://www.jqueryui.com) components.
+
+In order to use a widget, your view file would do the following:
+
+```php
+// Note that you have to "echo" the result to display it
+echo \yii\widgets\Menu::widget(['items' => $items]);
+
+// Passing an array to initialize the object properties
+$form = \yii\widgets\ActiveForm::begin([
+    'options' => ['class' => 'form-horizontal'],
+    'fieldConfig' => ['inputOptions' => ['class' => 'input-xlarge']],
+]);
+... form inputs here ...
+\yii\widgets\ActiveForm::end();
+```
+
+In the first example in the code above, the [[yii\base\Widget::widget()|widget()]] method is used to invoke a widget
+that just outputs content. In the second example, [[yii\base\Widget::begin()|begin()]] and [[yii\base\Widget::end()|end()]]
+are used for a
+widget that wraps content between method calls with its own output. In case of the form this output is the `<form>` tag
+with some properties set.
+