How can I better understand the level of service in symfony2 in php

I will try my best to explain the services, but Symfony2 docs does a better job than I can.

It has a core, a service is just a class. A class becomes a service when it is registered with Symfony2 Dependency Injection Container(or simply Containerfor brevity.) At this point, the class is part of the application service level.

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$emailSender = new EmailSender(new SmtpTransport());  
$emailSender->send($email)

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:

// YourApp/YourBundle/Resources/config/services.yml
services:
  smtp_transport:
    class: YourApp\YourBundle\Email\SmtpTransport

  email_sender:
    class: YourApp\YourBundle\Email\EmailSender
    arguments:
      - @smtp_transport

, ( Container):

$container->get('email_sender')->send($email);

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