Dynamics 365 is the "new" brand for all of Microsoft's business apps. Within that brand is Dynamics Business Central, which is basically a multi-tenant, Azure cloud hosted Dynamics NAV. Dynamics NAV as a brand will be retired, and it will be only Dynamics 365 Business Central moving into 2019, or at least that's the info we have now. My guess is that Microsoft is going to re-think this branding somewhat ... they may want to keep the 365 I'm fairly sure, but Dynamics itself could be replaced, and Business Central may not be the final answer.
Beyond the marketing, these collection of platforms offer a lot of true value. My guess it that the Office 365 momentum is going to start really impact D365 moving into 2019, since every D365 customer has so much opportunity with the Appsource and native apps in Office 365. It's actually a value story that virtually no other publisher can offer, and hopefully Microsoft is effective in communicating this message to customers in coming quarters.