What is SharePoint App Model?

The SharePoint App Model (now often referred to as SharePoint Add-ins or SharePoint Framework (SPFx) solutions for modern approaches) provided a way to extend SharePoint functionality without directly modifying the core SharePoint installation. It allowed developers to create and deploy custom solutions, such as web parts, applications, and services, that run within SharePoint or remotely, often leveraging web technologies like JavaScript, HTML, and REST APIs. This approach promoted greater stability, manageability, and security compared to older, more intrusive customization methods like farm solutions. They are used to build custom functionalities such as custom user interfaces, workflows, and data integrations within SharePoint.