Odoo comes with two editions. The first is the Community Edition, which is open source, and the second is the Enterprise Edition, which has licensing fees. Unlike other software vendors, Odoo Enterprise Edition is just a bunch of advance applications that adds extra features/apps in the Community Edition. Basically, Enterprise Edition runs on top of the Community Edition. The Community Edition comes under the Lesser General Public License v3.0 (LGPLv3) license and comes with all of the basic Enterprise resource planning (ERP) applications, such as sale, Customer Relationship Management (CRM), invoicing, purchase, website builder, and so on. Alternatively, Enterprise Edition comes with the Odoo Enterprise Edition License, which is a proprietary license. Odoo Enterprise Edition comes with advanced features such as full accounting, studio, Voice over Internet Protocol (VoIP), mobile responsive design, e-sign, marketing automation, and delivery and banking integrations. Enterprise Edition also provides you with unlimited bugfixes. The following diagram shows that Enterprise Edition depends on the Community Edition, which is why you need Community Edition to use Enterprise Edition:
You can see a full comparison of both editions here: https://www.odoo.com/page/editions.
Odoo has the largest number of community developers, which is why you will find a large number of third-party apps (modules) on the app store. Most of the free apps use an Affero General Public License version 3 (AGPLv3). You cannot use the proprietary license on your app if your application has dependencies on such apps. Apps with an Odoo proprietary license can be developed only on modules that have LGPL or other proprietary licenses.