Services in Drupal 8

In Drupal 8  service is any object managed by the services container.

Drupal 8 introduces the concept of services to decouple reusable functionality and makes these services pluggable and replaceable by registering them with a service container. As a developer, it is best practice to access any of the services provided by Drupal via the service container to ensure the decoupled nature of these systems is respected.
services are used to perform operations like accessing the database or sending an e-mail.

like below:

$connection = \Drupal::database();
$result = $connection->select('node', 'n')
  ->fields('n', array('nid'))
  ->execute()

you can create your own services,  like create common used function with in the class and use it any where, It works like global functions.

To create our own services, just create a services file like modulename.services.yml ,

services:
        drupalup_service.cow:
        class: Drupal\drupalup_service\CowService
        arguments: ['@current_user']

and create the callable class with functions which you like,
lets suppose we create a function 

public function whoIsYourOwner() {
    return $this->currentUser->getDisplayName();
  }

and use in anywhere like 

$aa = \Drupal::service('drupalup_service.cow');
kint($aa->whoIsYourOwner());

There are two methods to call the services,
1- Service location ( like \Drupal::database();)
2 - Dependency injection , (to inject as a dependency when create a service)

//Get a service from outside the current class:
$services = \Drupal::service('service_name');
//Equivalant , if you have a $container variable (also known as service container)
$service = $container->get('service_name');

How to call services within non service class with dependencies : 
It's achieved by create function in class, the create function( public function create($container){}) with in drupal 

fires automatically, it needs one argument $container from container interface,

get github code: https://github.com/Dharmend/drupalup_service