SharePoint 2016 removes the confusion by calling everything an app. Instead of a specialized calendar list, you now have a Calendar app. Need a discussion board? You now add the Discussion Board app.
Microsoft has also expanded the ability to develop custom developed solutions for SharePoint called add-ins. The result is that third parties or in-house developers can create add-ins for any purpose you can conceive. Imagine that you’re in Accounts Payable and you use SharePoint 2016 to manage your invoice documents. You might request an add-in from your IT department to route documents through the various approvals before finally being submitted to the payment system.
If you use SharePoint Online, you can browse, purchase, and install apps and add-ins from the SharePoint Store without leaving SharePoint. It’s similar to using the App Store on an iPhone or Google Market on an Android phone to buy and download an app.
Many SharePoint power users are comfortable with the concept of a data container called a list. You can still start with a blank list and add functionality as required. Nothing has changed in this ability. You are now just creating your own custom app by starting with the Custom List app. After you give your new app a name, it appears alongside all the other apps.