Before 1st November 2016, Microsoft Dynamics was split into two systems: Enterprise Resource Planning (ERP) and Customer Relationship Management (CRM). ERP is built on Dynamics NAV, AX and GP, whereas, CRM is built on Dynamics CRM.
Dynamics 365 combines the capabilities of these two systems. Unlike ERP and CRM built on layers of complex databases, Dynamics 365 has a common data model making it easier to extend and upgrade.