I have started a trial instance of Dynamics 365. It has sales, service, and marketing. I'd like to add an "area"(d) or tab for my own app and if possible remove those.
How can I remove and add tabs? Also, what are those called, in salesforce they are called apps.
Seeing as you mentioned Dynamics 365 you should be able to use the new SiteMap designer to add a new area to the SiteMap.
Navigate to Settings -> Customisations -> Customise the system
In there click on Client Extensions.
Select the sitemap in the list and you should have an edit button.
Rather than documenting it here there are some useful resources and training on Microsoft's site to guide you through this: see here