d8block.php

<?php
namespace Drupal\hello\Plugin\Block;

use Drupal\Core\Block\BlockBase;

/**
 * Class HelloBlock
 * @package Drupal\hello\Plugin\Block
 *
 * @Block(
 *   id = "leon_hello_block",
 *   admin_label = @Translation("Leon's hello block"),
 *   category = @Translation("Leon")
 * )
 */
class HelloBlock extends BlockBase {
    /**
     * Builds and returns the renderable array for this block plugin.
     *
     * @return array
     *   A renderable array representing the content of the block.
     *
     * @see \Drupal\block\BlockViewBuilder
     */
    public function build()
    {
        return array(
            '#title' => 'Hello world!',
            '#markup' => 'This is the content'
        );
    }
}