Office Add-ins formerly Web Apps for Office. Are apps that run inside an Office application and can interact with the contents of the Office document using the rich JavaScript API, Office Add-ins are going to really popular with Office 2013, 2016 and Office365
So what is Office Add In?
Simply put an Office Add-in is a web app that you can host anywhere including your DreamSpark Azure Subscription which offers free web site. If you haven't activated your Free Azure Subscription follow this guide.
Building your first Office Add-In
Using VS Code to Develop Your Office Add-in!
VS Code is a great tool to help you develop your custom Office Add-ins regardless if they are for Outlook, Word, Excel, PowerPoint and run in the web clients, Windows clients, iOS clients or on OS X! The following site has a full step by step walkthrough https://code.visualstudio.com/Docs/runtimes/office
Using Visual Studio 2015 to Develop your Office Add-in!
http://dev.office.com/getting-started/addins you can watch the following video tutorial
What development languages are supported
You can. You can use pure HTML, Angular, Ember, React, Aurelia, additionally VS Code has great support for TypeScript!
Hosting Your Office Add-in Development
Office Add-ins must be served via HTTPS; the Office application will not load a web app as an Add-in if it is HTTP. Office Add-ins must be served via HTTPS; the Office application will not load a web app as an Add-in if it is HTTP. To develop, debug and host the Add-in locally, you need a way to create and serve a web app locally using HTTPS.
So once you have built your add-in how do end users actually use it?
Install the Add-in
Office Add-ins must be installed, or registered, with the Office application in order to load. This is done using the manifest.xml
How to Side Load Add-ins
Here is a quick example of adding a Mail Add-ins
Mail add-ins can be installed within the Outlook Web App. Browse to your Outlook Web App (https://mail.office365.com) and login. Once logged in, click the gear icon in the top-right section and select Manage add-ins:
On the Manage add-ins page, select the + icon and then select Add from a file.
Deploying Add-ins to the Office 365 Tenancy's App Catalog
All Office Add-ins (including Mail Add-ins) can be installed from your Office 365 tenancy's App Catalog site. Log in to your Office 365 Portal. In the left-hand navigation, towards the bottom, select the Admin / SharePoint option:
From the SharePoint Admin Center, select the Apps option in the left-hand menu and then select the App Catalog. On the App Catalog page, select the Apps for Office option and upload the manifest.xml file.
Install Content & Task Pane Add-ins in Word / Excel / PowerPoint
Depending on the type of Add-in you created, you can add it to one of the Office applications. Task Pane and Content Add-ins can be installed in Word, Excel & PowerPoint. Mail Add-ins can be installed in Outlook.
To install an Add-in within an Office application, select the Insert tab and click the Office Add-ins button, as shown here using the Excel Web App:
Using the Office Add-ins dialog you can select Add-ins you've uploaded to your Office 365 tenancy's App Catalog (listed under My Organization) or acquire Add-ins from the Office Store.
Conclusion
Office Add-ins are a great way of providing extra features to Microsoft Office products.
Office Add-ins are the perfect opportunity to showcase your development skills.
Office Add-ins development experiences will be a highly desired skill set for potential employers.
Software vendors who which to make available their services or products can use Add-ins to make their services native within Microsoft Office.
A number of vendors are already developing add-ins for office which are now available to install at https://store.office.com/