PrestaShop modules are addons you can install in your PrestaShop back office to add functionality to your store. Think of modules as extensions to your online store. Modules work with your PrestShop installation to make it possible to do more things or do things easier in your store.
Pre-Installed PrestaShop Modules
The full installation of PrestaShop comes with over 100 modules you can easily install/uninstall and configure for your PrestaShop site. The files for these modules are already includes with your PrestaShop installation and simply need to be installed and configured. For simplicity's sake, we will refer to these as pre-installed modules and we cover how to check for an install those modules in our tutorial on enabling and configuring pre-installed modules in PrestaShop.
Finding Additional PrestaShop Modules
The official PrestaShop Marketplace: http://addons.prestashop.com is also a great source for PrestaShop modules. In the PrestaShop Addons store you can browse by module categories or search for modules.
You're not limited to the PrestShop Addons store for finding modules for your PrestaShop store. Performing a search in your preferred search engine for phrases like: prestashop module or prestashop addon will return a large number of resources for finding new PrestaShop modules. You can also add words related to what you want the module to do. For example, if I wanted to add some social networking to my store, I might search for: prestashop module social.
Once you've found a module that you want to upload and add to your PrestaShop store, download the module to your local computer. In our tutorial on uploading and installing a module in the PrestaShop back office, we walk you through how to upload and install a PrestaShop module you have found either in the PrestaShop marketplace or eslewhere online.