Diffrence b/w SharePoint APPs and Add-ins

There is no difference in these two.

Earlier it used to be called as App, in recent years Microsoft has changed the nomenclature to Add-in.

 

So in Add-in you can have SharePoint Hosted add-in or Provider Hosted Add-in. If you want to customize something in SharePoint you can create a add-in.

 

But these is also old technique.

 

Now its all SPFx - The SharePoint Framework (SPFx) is a page and web part model that provides full support for client-side SharePoint development, easy integration with SharePoint data, and support for open source tooling