What is Apps in sharepoint 2013

Apps

Apps are new type of solution that help to extend MS Office 2013 product features to fulfill business demands, built on HTML, CSS, JavaScript, REST, etc.
An App for Office is essentially a webpage that is hosted inside an Office client/web application.
SharePoint Administration Center control for managing Apps in Microsoft Office 365 – SharePoint 2013 Online.
Components of an App for Office are XML manifest file and webpage.
The manifest defines configuration, settings, logic and points to the webpage having App UI.
Types of App.

• App for SharePoint
• Cloud App Model
• self-contained sections having technologies such as HTML5, JavaScript 
• Light imprint as they don't actually install on the host server.
• Don't overload a SharePoint site with unnecessary API calls. 
• Cloud App Model offers users to download app and install on their sites. 
• Task pane App for Office
• Content App for Excel
• Mail App for Office

App Part:

Its a type of Web Part that is represented by the ClientWebPart class.
Way by which an App for SP 2013 can be appeared is through an App part
An App Part is essentially a wrapper for an IFrame that host a page of the App & act like Webpart
App part can have custom properties that users can set in a tool part.
It has following parts:

• ClientWebPart.aspx - In a SharePoint Hosted App, this will act as your App Part Interface.
• Default.aspx - is the start page for the App. This will provide the full page App experience, which will be the landing page after the deployment of the App.
• App.css – This will contain the default styling and based on the requirement, we can use the same CSS file to add any custom styles.
• App.js – In this JavaScript file, it contains the required JavaScript and has few lines of pre-generated code to start your SharePoint App development.

Read about "App Catalog"


Use of NAPA n office 365