Working With SharePoint 2013 APP - Introduction

Apps for SharePoint or SharePoint Add-ins:

First, a quick background: 

SharePoint 2010 introduced the sandboxed concept, the predecessor to SharePoint Apps, but it offered limited access for developers at the site-collection level. It had many issues and had to work under several code access security (CAS) policies. What grew out of that was the need for a new concept that would allow developers to become more productive, 

efficient and have more freedom and, thus, SharePoint Apps.

One of the important things to note is that a SharePoint App creates a sub-site of the site where the app is installed and supports creating lists, defining properties and so on.

Microsoft set out to create an extensibility point in SharePoint 2013 that would allow customers to build their own solutions for SharePoint without hurting the hosted model whenever a customer’s code was found to be ‘executionally challenged’. They came up with a model called a SharePoint App, which is simply a solution with no SharePoint server side code.

Here are some important notes to be made on SharePoint Apps when compared with farm solutions:

1. App code never runs within the SharePoint host environment.

2. Apps have two scopes: They can work within the scope of a SharePoint site, as well as with a special SharePoint site        called the App Catalog.

3. No down time is required for the whole environment when an app is being upgraded.

https://visualstudiomagazine.com/articles/2015/04/30/sharepoint-hosted-app-visual-studio.aspx