Microsoft Dynamics 365 Customer Engagement is the group of applications that manage everything facing your customers: attracting them, selling to them, supporting them and servicing them on site. It is what most people mean when they say Dynamics 365 CRM, and Microsoft uses both names for the same set of products.
The important thing is that it is a suite, not a single product. Sales, Customer Service, Field Service and Customer Insights are licensed and deployed separately, but they all write to the same underlying data platform. A contact created by a marketing campaign is the same record a salesperson works, the same record a service agent opens a case against, and the same record an engineer sees on a job.
That shared foundation is the practical difference between Dynamics 365 and stitching together separate best-of-breed tools. There is no integration to build and no synchronisation to maintain, because there is only ever one version of the customer. It also means you can start with one application and add the next when the business is ready, rather than buying a platform you grow into slowly.