Hi Alejandro.

Local Modules: Modules that are stored on your server, you can set the folder addresses on config file under addons_path

Apps (if it's related to online apps): Modules that are on apps.odoo.com (these modules could exist or not on your server)

Apps (Local Apps): These are modules that are on your local server but only the ones that have "application" on their manifest file. These modules could be or not custom modules. Usually these modules are the base ones that would install other sub-modules that are not set as applications. For example (Sales, Purchase, Accounting)

Ideally Apps means one complete package/module, not necessarily the standard modules developed by Odoo.

For example say CRM, SALES

When you install CRM, the complete functionalities/workflow is install, same happens with SALES, however there can be modules say an  extended version of those Apps say "sale_crm", now these will be of supporting modules rather than a complete one, hence they are simply treated as modules.

Local Modules, are the custom modules.

Local module is module in your local storage, it mix between your custom module and module who was created by odoo. but on apps its only contain module create by odoo.