How to create custom module in Drupal 8

In Drupal 8, we should keep the custom or contributed modules under modules folder in the root directory.

First of all I am going to create a folder for custom module that is "Custom Module" with a machine name of custom_module.

Start the module by creating a folder within the Drupal installation and path is: /modules/custom/custom_module or /sites/all/modules/custom/custom_module.

custom_module.info.yml : Now Creating the "custom_module.info.yml" in the root of the custom_module directory.

1. File: modules/custom/custom_module/custom_module.info.yml:

File: modules/custom/custom_module/custom_module.info.yml:
name: My custom module name
description: Creates a page showing "Hello World".
package: Custom

type: module
core: 8.x

2. File: modules/custom/custom_module/custom_module.module: This file is required but can be empty.

<?php

/**
 * @file
 * Hello module.
 */

3. File: modules/custom/custom_module/custom_module.routing.yml: This file is required but can be empty.

hello_world:
  path: '/hello_world'
  defaults:
    _title: 'Hello Title'
    _controller: '\Drupal\custom_module\HelloController::content'
  requirements:
    _permission: 'access content'

4. File: modules/custom/custom_module/src/HelloController.php:

In Drupal 8, classes will be autoloaded based on the PSR-4 namespacing convention.

In core, the PSR-4 'tree' starts under core/lib/.

In modules, including contrib, custom and those in core, the PSR-4 'tree' starts under modulename/src.

Defining a class in our module's .module file is only possible if the class does not have a superclass which might not be available when the .module file is loaded. It's best practice to move such classes into a PSR-4 source directory.

<?php
/**
 * @file
 * Contains \Drupal\custom_module\HelloController.
 */

namespace Drupal\custom_module;


use Drupal\Core\Controller\ControllerBase;


class HelloController extends ControllerBase {
  public function content() {
    return array(
        '#markup' => '' . t('Hello there!') . '',
    );
  }
}

Here Namespaces are a way of organizing codebases. Namespaces documentation on php.net.

5. Go to module list page and enable the custom module and visit the path.