Apps for SharePoint overview

Apps for SharePoint are easy-to-use, lightweight web applications that integrate popular web standards and technologies to extend the capabilities of a SharePoint website. This topic helps you understand the basic concepts about how to build apps in SharePoint 2013.

Why build apps for SharePoint?

There are many great reasons to build an app for SharePoint:

• A familiar programming model, plus popular technologies and languages
• Multiple options for hosting
• A familiar web experience, yet more flexible
• Integrates with apps for Office
• Lets you reach users through the Office Store and the App Catalog

Building an app for SharePoint

To build your app for SharePoint, you must do the following:

• Configuring an app
• Deciding where the app’s code runs (hosting options)
• Communicating with SharePoint using SharePoint APIs
• Packaging and distributing an app

Deciding where the app’s code runs (hosting options)

You decide where the app’s code runs by choosing the hosting option —in SharePoint, in the cloud, or a mix of SharePoint and cloud.

• Provider-hosted apps: You or your app purchaser’s IT department must host a provider-hosted app on a dedicated server or third-party hosting service. Depending on your choice, these apps run either on your server or in the cloud.
• SharePoint-hosted apps: When the app is hosted in SharePoint itself, the code is only in HTML and JavaScript and is hosted by SharePoint.
• Hybrid apps (components in both SharePoint and the cloud): In these apps, SharePoint components run in SharePoint while components hosted in the cloud run in the cloud.

Communicating with SharePoint using SharePoint APIs

Apps show their power when they use SharePoint APIs to connect and integrate with SharePoint features—search, workflow, social networking, taxonomy, user profiles, BCS, and more. This lets them read documents, do searches, connect people, and much more. SharePoint APIs include the following:

• REST APIs and JavaScript APIs
• Managed APIs (.NET)

To start working with SharePoint APIs, see How to: Complete basic operations using SharePoint 2013 REST endpoints, How to: Complete basic operations using JavaScript library code in SharePoint 2013, and Basic operations with the SharePoint .NET client object model.

Before you call SharePoint APIs, you must authenticate to SharePoint and get user consent or authorization. For more information, see Data access options for apps in SharePoint 2013 and App permissions in SharePoint 2013.

Packaging and distributing an app

Apps for SharePoint are distributed as an app package, and the components it contains depend on where your app is hosted. A provider-hosted app package might contain only the app manifest if the app is hosted on a dedicated server or third-party hosting service. A SharePoint-hosted app package might contain some SharePoint-related components and the app manifest. In addition, app packages can include components such as apps for Office. For more information, 

If you use "Napa" Office 365 Development Tools or Visual Studio 2012 to build apps and deploy directly from them, the tools let you publish your app into a file that has an .app extension. You can then distribute the app in one of two ways:

• Apps that are meant to run within a particular organization can be deployed using the App Catalog.
• Apps for the Office Store can be submitted via the Microsoft Seller Dashboard. Always remember to add licensing checks to the app. 

Reference

http://msdn.microsoft.com/EN-US/library/office/fp179930(v=office.15).aspx