What is a block class in Magento 2? Explain with a code example.

A block class in Magento 2 is a class that represents a block of content that can be rendered on a page. These blocks typically contain the HTML markup and logic for displaying specific pieces of content on a page, such as a product list or a navigation menu.

For example, the following code creates a block class for displaying a list of products on a category page:

class ProductList extends \Magento\Framework\View\Element\Template
{
    protected $_productCollectionFactory;

    public function __construct(
        \Magento\Framework\View\Element\Template\Context $context,
        \Magento\Catalog\Model\ResourceModel\Product\CollectionFactory $productCollectionFactory,
        array $data = []
    ) {
        $this->_productCollectionFactory = $productCollectionFactory;
        parent::__construct($context, $data);
    }

    public function getProductCollection()
    {
        $categoryId = $this->getData('category_id');
        $productCollection = $this->_productCollectionFactory->create();
        $productCollection->addCategoryFilter($categoryId);
        return $productCollection;
    }
}

In this example, the ProductList class extends the \Magento\Framework\View\Element\Template class, which provides the basic functionality for rendering blocks in Magento 2. The class also has a constructor that accepts a factory for creating product collections, and a getProductCollection method that returns a collection of products filtered by a specific category ID. This block class can then be used in a template file to display a list of products for a specific category.