A SharePoint Add-in is a self-contained PIECE of functionality that expands SharePoint websites' capabilities to tackle a specific business need. Custom code that runs on SharePoint servers is not present in add-ins. Instead, all custom logic is moved "up" to the cloud, "down" to client computers, or "over" to an on-premises server that is not part of the SharePoint farm or subscription. SharePoint administrators may rest assured that the add-in will neither HARM their servers or degrade the performance of their SharePoint Online websites by keeping custom code off of them. One of the various client APIs offered in SharePoint allows business logic in a SharePoint Add-in to access SharePoint data. Which API you pick for your add-in is determined by a number of additional design considerations. A SharePoint Add-in can include almost all major types of SharePoint components, such as pages, lists, workflows, custom content types, LIST templates, web parts, and more. Host webs are SharePoint websites on which SharePoint Add-ins are installed and from which users launch them. The SharePoint components, on the other hand, are usually located in an add-in web, which is a specific child web of the host web. 

SharePoint Add-ins can be integrated into a SharePoint site in a variety of ways :

• As a full-page immersive experience with the appearance and feel of a SharePoint page.

• To expose an iframe element that contains the add-in as part of a webpage, utilizing a PARTICULAR type of control called an add-in part.

• As UI commands for lists, documents, and more than augment ribbons and menus.

On the Site Contents page of the SharePoint website, any SharePoint Add-ins that users install are given a tile. The add-in is launched by clicking the tile.

An add-in manifest—an XML file that describes the add-fundamental in's properties, where it runs, and what SharePoint should do when the add-in starts—is used to SET up a SharePoint Add-in. The manifest can indicate what languages the add-in supports, what SharePoint services and capabilities it relies on, and what permissions the add-in requires on the host web, among other things. (SharePoint Add-ins have complete control over their own web.