diff --git a/components/dependency_injection/definitions.rst b/components/dependency_injection/definitions.rst
index bd13b766b43..8fab146806d 100644
--- a/components/dependency_injection/definitions.rst
+++ b/components/dependency_injection/definitions.rst
@@ -4,6 +4,9 @@
 Working with Container Service Definitions
 ==========================================
 
+Service definitions are the instructions describing how the container should
+build a service. They are not the actual services used by your applications
+
 Getting and Setting Service Definitions
 ---------------------------------------
 
@@ -32,7 +35,17 @@ with these methods and make changes to it these will be reflected in the
 container. If, however, you are creating a new definition then you can add
 it to the container using::
 
-    $container->setDefinition($id, $definition);
+    use Symfony\Component\DependencyInjection\Definition;
+
+    $definition = new Definition('Acme\Service\MyService');
+    $container->setDefinition('acme.my_service', $definition);
+
+.. tip::
+
+    Registering service definitions is so common that the container provides a
+    shortcut method called ``register()``::
+
+        $container->register('acme.my_service', 'Acme\Service\MyService');
 
 Working with a Definition
 -------------------------
@@ -40,26 +53,50 @@ Working with a Definition
 Creating a New Definition
 ~~~~~~~~~~~~~~~~~~~~~~~~~
 
-If you need to create a new definition rather than manipulate one retrieved
-from the container then the definition class is :class:`Symfony\\Component\\DependencyInjection\\Definition`.
+In addition to manipulating and retrieving existing definitions, you can also
+define new service definitions with the :class:`Symfony\\Component\\DependencyInjection\\Definition`
+class.
 
 Class
 ~~~~~
 
-First up is the class of a definition, this is the class of the object returned
-when the service is requested from the container.
+The first optional argument of the ``Definition`` class is the fully qualified
+class name of the object returned when the service is fetched from the container::
 
-To find out what class is set for a definition::
+    use Symfony\Component\DependencyInjection\Definition;
+
+    $definition = new Definition('Acme\Service\MyService');
+
+If the class is unknown when instantiating the ``Definition`` class, use the
+``setClass()`` method to set it later::
 
-    $definition->getClass();
+    $definition->setClass('Acme\Service\MyService');
 
-and to set a different class::
+To find out what class is set for a definition::
 
-    $definition->setClass($class); // Fully qualified class name as string
+    $class = $definition->getClass();
+    // $class = 'Acme\Service\MyService'
 
 Constructor Arguments
 ~~~~~~~~~~~~~~~~~~~~~
 
+The second optional argument of the ``Definition`` class is an array with the
+arguments passed to the constructor of the object returned when the service is
+fetched from the container::
+
+    use Symfony\Component\DependencyInjection\Definition;
+
+    $definition = new Definition(
+        'Acme\Service\MyService',
+        array('argument1' => 'value1', 'argument2' => 'value2')
+    );
+
+If the arguments are unknown when instantiating the ``Definition`` class or if
+you want to add new arguments, use the ``addArgument()`` method, which adds them
+at the end of the arguments array::
+
+    $definition->addArgument($argument);
+
 To get an array of the constructor arguments for a definition you can use::
 
     $definition->getArguments();
@@ -69,12 +106,16 @@ or to get a single argument by its position::
     $definition->getArgument($index);
     // e.g. $definition->getArgument(0) for the first argument
 
-You can add a new argument to the end of the arguments array using::
+The argument can be a string, an array or a service parameter by using the
+``%parameter_name%`` syntax::
 
-    $definition->addArgument($argument);
+    $definition->addArgument('%kernel_debug%');
 
-The argument can be a string, an array, a service parameter by using ``%parameter_name%``
-or a service id by using::
+If the argument is another service, don't use the ``get()`` method to fetch it,
+because it won't be available when defining services. Instead, use the
+:class:`Symfony\\Component\\DependencyInjection\\Reference` class to get a
+reference to the service which will be available once the service container is
+fully built::
 
     use Symfony\Component\DependencyInjection\Reference;
 
@@ -139,4 +180,3 @@ the service itself gets loaded. To do so, you can use the
 
 Notice that Symfony will internally call the PHP statement ``require_once``,
 which means that your file will be included only once per request.
-
diff --git a/components/dependency_injection/introduction.rst b/components/dependency_injection/introduction.rst
index 42c526652fd..a4ad2e93dac 100644
--- a/components/dependency_injection/introduction.rst
+++ b/components/dependency_injection/introduction.rst
@@ -104,7 +104,9 @@ like this::
         // ...
     }
 
-Then you can register this as a service as well and pass the ``mailer`` service into it::
+When defining the ``newsletter_manager`` service, the ``mailer`` service does
+not exist yet. Use the ``Reference`` class to tell the container to inject the
+``mailer`` service when it initializes the newsletter manager::
 
     use Symfony\Component\DependencyInjection\ContainerBuilder;
     use Symfony\Component\DependencyInjection\Reference;