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Only the three first are necessary for a basic module: the main file, the cache configuration file (which is autogenerated by PrestaShop anyway) and the icon file. All the other ones can be used if necessary, but a module can work without them.
The module can also as many other files and folders as necessary: /css, /img, /js, etc.

The PrestaShop coding convention

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<?php	
if (!defined('_PS_VERSION_')) {
  exit;
} 

This checks for the existence of an always-existing PrestaShop constant (its version number), and if it does not exist, it stops the module from loading. The sole purpose of this is to prevent malicious visitors to load this file directly.

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<?php
if (!defined('_PS_VERSION_')) {
  exit;
}

class MyModule extends Module
{
}

...

At this stage, if you place the module's folder on the /modules folder, the module can already be seen in the "Modules" page in the back - office, in the "Other modules" section – albeit with no real name nor thumbnail.

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<?php
if (!defined('_PS_VERSION_')) {
  exit;
}

class MyModule extends Module
{
  public function __construct()
  {
    $this->name = 'mymodule';
    $this->tab = 'front_office_features';
    $this->version = '1.0.0';
    $this->author = 'Firstname Lastname';
    $this->need_instance = 0;
    $this->ps_versions_compliancy = array('min' => '1.6', 'max' => _PS_VERSION_);	
    $this->bootstrap = true;

    parent::__construct();

    $this->displayName = $this->l('My module');
    $this->description = $this->l('Description of my module.');

    $this->confirmUninstall = $this->l('Are you sure you want to uninstall?');

    if (!Configuration::get('MYMODULE_NAME'))		 {
      $this->warning = $this->l('No name provided');
    }
  }
}

Let's examine each line from this first version of the MyModule class...

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• 'name' attribute. This attributes serves as an internal identifier. The value MUST be the name of the module's folder. Do not use special characters or spaces, and keep it lower-case.
• 'tab' attribute. The title for the section that shall contain this module in PrestaShop's back - office modules list. You may use an existing name, such as seo, front_office_features or analytics_stats, or a custom one. In this last case, a new section will be created with your identifier. We chose "front_office_features" because this first module will mostly have an impact on the front-end.
  

  Here is the list of available "Tab" attributes, and their corresponding section in the "Modules" page:

  "Tab" attribute	Module section
  administration	Administration
  advertising_marketing	Advertising & Marketing
  analytics_stats	Analytics & Stats
  billing_invoicing	Billing & Invoices
  checkout	Checkout
  content_management	Content Management
  dashboard	Dashboard
  emailing	E-mailing
  export	Export
  front_office_features	Front Office Features
  i18n_localization	I18n & Localization
  market_place	Market Place
  merchandizing	Merchandizing
  migration_tools	Migration Tools
  mobile	Mobile
  others	Other Modules
  payments_gateways	Payments & Gateways
  payment_security	Payment Security
  pricing_promotion	Pricing & Promotion
  quick_bulk_update	Quick / Bulk update
  search_filter	Search & Filter
  seo	SEO
  shipping_logistics	Shipping & Logistics
  slideshows	Slideshows
  smart_shopping	Smart Shopping
  social_networks	Social Networks

• 'version' attribute. The version number for the module, displayed in the modules list. It is a string, so that you may use such variation as "1.0b", "3.07 beta 3" or "0.94 (not for production use)".
• 'author' attribute. This is displayed as-is in the PrestaShop modules list.

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• need_instance. Indicates whether to load the module's class when displaying the "Modules" page in the back-officeback office. If set at 0, the module will not be loaded, and therefore will spend less resources to generate the "Modules" page. If your module needs to display a warning message in the "Modules" page, then you must set this attribute to 1.
• ps_versions_compliancy. Indicates which version of PrestaShop this module is compatible with. In the example above, we explicitly write that this module will only work with PrestaShop 1.5.x, and no other major version.
• bootstrap. Indicates that the module's template files have been built with PrestaShop 1.6's bootstrap tools in mind – and therefore, that PrestaShop should not try to wrap the template code for the configuration screen (if there is one) with helper tags.

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$this->displayName = $this->l('My module');
$this->description = $this->l('Description of my module.');

$this->confirmUninstall = $this->l('Are you sure you want to uninstall?');

if (!Configuration::get('MYMODULE_NAME'))	 {
    $this->warning = $this->l('No name provided.');  
}

These lines respectively assign:

• A name for the module, which will be displayed in the back - office's modules list.
• A description for the module, which will be displayed in the back - office's modules list.
• A message, asking the administrator if he really does want to uninstall the module. To be used in the installation code.
• A warning that the module doesn't have its MYMODULE_NAME database value set yet (this last point being specific to our example, as we will see later).

The constructor method is now complete. You are free to add more to it later if necessary, but this the bare minimum for a working module.

Now go to your back - office, in the Modules page: the module is visible in the modules list, with its information displayed – and no icon for now.

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public function install()
{
  if (!parent::install()) {
    return false;
  return}

  return true;
}

In this first and extremely simplistic incarnation, this method does the minimum needed: return true returned by the Module class' install() method, which returns either true if the module is correctly installed, or false otherwise. As it is, if we had not created that method, the superclass' method would have been called instead anyway, making the end result identical.
Nevertheless, we must mention this method, because it will be very useful once we have to perform checks and actions during the module's installation process: creating SQL tables, copying files, creation configuration variables, etc.

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public function install()
{
  if (Shop::isFeatureActive()) {
    Shop::setContext(Shop::CONTEXT_ALL);
  }

  if (!parent::install() ||
    !$this->registerHook('leftColumn') ||
    !$this->registerHook('header') ||
    !Configuration::updateValue('MYMODULE_NAME', 'my friend')
  ) {
    return false;
  }
return
  return true;
}

If any of the lines in the testing block fails, the method returns false and the installation does not happen.

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public function uninstall()
{
  if (!parent::uninstall()) {
    return false;
  }
  
  return true;
}

Building on this foundation, we want an uninstall() method that would delete the data added to the database during the installation ( MYMODULE_NAME configuration setting). This method would look like this:

public function uninstall()
{
  if (!parent::uninstall() ||
    !Configuration::deleteByName('MYMODULE_NAME')
  ) {
    return false;
  }

  return true;
}

The Configuration object

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if (Shop::isFeatureActive()) {
  Shop::setContext(Shop::CONTEXT_ALL);
}

As said earlier, here we check that the Multistore feature is enabled, and if so, set the current context to all shops on this installation of PrestaShop.

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To put the finishing touch to this basic module, you should add an icon, which will be displayed next to the module's name in the back - office modules list.
In case your module is made for a prominent service, having that service's logo visible brings trust.
Make sure you do not use a logo already used by one of the native modules, or without authorization from the owner of the logo/service.

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Now that all basics are in place, reload the back - office's "Modules" pages, in the "Front - office features" section, you should find your module. Install it (or reset it if it is already installed).

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