First of all, it’s important to clarify that Microsoft currently markets two (very) different Office versions: Office Professional and Office 365.
Office Professional is actually the “old” desktop Office version (the one that everyone is familiar with, for many years), while Office 365 is the web version (or cloud version) of Microsoft Office. Microsoft Office 365 (initially called Office Online or Office Live) was actually around for some years too, but – with the release of Windows RT for Surface & other mobile platforms – it will become more and more popular. Some Office 365 subscriptions also include the desktop Office application versions, such as Outlook or Word – to avoid further confusions, in this article we refer to Office 365 as the web based version.
Both Office versions feature the same set of products: Word, Excel, Outlook, etc. While Office Professional runs from/on your computer, Office 365 runs on the web, on Microsoft servers (office365.microsoft.com). You can install add-ins in Office Professional 2013 (just like you did on the previous Office versions), but they are not available for Office 365 (unless you download the desktop application versions available under some Office 365 subscriptions). Starting with Office 2013, Microsoft introduced “apps” for Office 365, so developers can create “add-ins” (now called “apps”) for this Office cloud version.
To put it very simple:
– if you are using Outlook 2013 Professional (part of Office 2013 Professional, Office 2013 Professional Plus or as a desktop Outlook version of Office 365), you can install Outlook add-ins;
– if you are using web based Outlook 365 or Office 2013 on Windows RT devices, you can install Outlook apps (and not add-ins).
Probably as part of the Windows RT marketing efforts, Microsoft discarded the old Office Marketplace (where you could find regular add-ins for Outlook, Word, etc) and replaced it with the Office Store that only hosts apps! So you can not use the Office Store to look for Outlook 2013 Pro add-ins: they are just not there anymore.
From a functionality point of view, there is a big difference between apps and add-ins. Outlook add-ins install on your computer and have greater access to Windows & Outlook resources, allowing add-in developers to provide far more features comparing to Outlook apps. On the other hand, Outlook apps are basically mini-applications that can extend your Office functionality mainly with web oriented features (like integrating Bing search with Outlook).
Regarding our Outlook add-ins, they are typical Outlook add-ins and so they can only run on the desktop version of Outlook (2013, 365 for desktop or older Outlook versions). We do not plan to make our add-ins available as Outlook apps simply because we can not technically develop similar features for the Outlook cloud version.