Difference between ObjectManager and EntityManager in Symfony2?

What's the difference between Doctrine\Common\Persistence\ObjectManager and Doctrine\ORM\EntityManager when using it in a custom form type?

I can get the respository using both $this->em->getRepository() and $this->om->getRepository().

class MyFormType extends \Symfony\Component\Form\AbstractType
{

    /**
     * @var Doctrine\ORM\EntityManager
     */
    protected $em;

    public function __construct(Doctrine\ORM\EntityManager $em)
    {
        $this->em = $em;
    }

 }

Instead of:

class MyFormType extends \Symfony\Component\Form\AbstractType
{

    /**
     * @var Doctrine\Common\Persistence\ObjectManager
     */
    protected $om;

    public function __construct(Doctrine\Common\Persistence\ObjectManager $om)
    {
        $this->om = $om;
    }

 }