There are two kinds of SharePont Add-ins.

1- SharePoint Hosted Add-in:

There are no server side code in case of SharePoint-hosted add-in. All business logic in a SharePoint-hosted add-in USES JavaScript either directly on a custom page or in a JavaScript file that is referenced from a custom page.

Custom pages in a SharePoint-hosted add-in are generally ASP.NET pages (ASPX) and they can declaratively reference ASP.NET and in-the-box SharePoint controls, but there can be no code behind. However, you can customize the SharePoint controls using a client-side rendering option and custom JavaScript.

The JavaScript in SharePoint-hosted add-ins can access data and resources that are outside of the add-in web by using either of two techniques for safely working around the browser's same origin policy: a special JavaScript cross-domain library or a specific JavaScript WebProxy class. Using these techniques a SharePoint-hosted add-in can work with data on the host web, its parent subscription.

SharePoint-hosted add-ins consist almost entirely of SharePoint components in an add-in web. A user can run a SharePoint-hosted add-in from a tile on the Site Contents page of the SharePoint website to which it's installed.

2- Provider Hosted Add-in:

Any SharePoint component that can be in a SharePoint-hosted add-in can also be in a provider-hosted add-in. But provider-hosted add-ins are distinguished from SharePoint-hosted add-ins because they include at least one remote component; such as a web application, service, or DATABASE that is hosted externally from the SharePoint farm or SharePoint Online subscription. This could be a server in the same corporate network as a SharePoint farm or a cloud service. The external components can be hosted on any web hosting STACK, including the Linux, Apache, MySQL, PHP (LAMP) stack. 

When the remote components are implemented with .NET, the managed code SharePoint Client-Side Object Model (CSOM) library is available.

For remote components that are not based on .NET, there is a set of REST/OData APIs that can be used to access SharePoint data. These can also be used from a .NET client if you prefer working with an OData interface.

There are two kinds of SharePont Add-ins.

1- SharePoint Hosted Add-in:

There are no server side code in case of SharePoint-hosted add-in. All business logic in a SharePoint-hosted add-in uses JavaScript either directly on a custom page or in a JavaScript file that is referenced from a custom page.

Custom pages in a SharePoint-hosted add-in are generally ASP.NET pages (ASPX) and they can declaratively reference ASP.NET and in-the-box SharePoint controls, but there can be no code behind. However, you can customize the SharePoint controls using a client-side rendering option and custom JavaScript.

The JavaScript in SharePoint-hosted add-ins can access data and resources that are outside of the add-in web by using either of two techniques for safely working around the browser's same origin policy: a special JavaScript cross-domain library or a specific JavaScript WebProxy class. Using these techniques a SharePoint-hosted add-in can work with data on the host web, its parent subscription.

SharePoint-hosted add-ins consist almost entirely of SharePoint components in an add-in web. A user can run a SharePoint-hosted add-in from a tile on the Site Contents page of the SharePoint website to which it's installed.

2- Provider Hosted Add-in:

Any SharePoint component that can be in a SharePoint-hosted add-in can also be in a provider-hosted add-in. But provider-hosted add-ins are distinguished from SharePoint-hosted add-ins because they include at least one remote component; such as a web application, service, or database that is hosted externally from the SharePoint farm or SharePoint Online subscription. This could be a server in the same corporate network as a SharePoint farm or a cloud service. The external components can be hosted on any web hosting stack, including the Linux, Apache, MySQL, PHP (LAMP) stack. 

When the remote components are implemented with .NET, the managed code SharePoint Client-Side Object Model (CSOM) library is available.

For remote components that are not based on .NET, there is a set of REST/OData APIs that can be used to access SharePoint data. These can also be used from a .NET client if you prefer working with an OData interface.