I have not really explored the apps for SharePoint 2013, and to my surprise there are quite many apps available now in the market. If you would like to explore, you can follow this link.
If you wonder what apps in SharePoint 2013 means. Official explanation is below:
Apps are mini applications that extend what you can do with the new version of Office and SharePoint 2013. For example, with apps you can use Wikipedia without leaving Word or get directions and maps right in Outlook. Apps are available for Word, Excel, Outlook, Project, PowerPoint, and SharePoint 2013.
So it is something like an add-on to your existing SharePoint that let you download the app from the cloud and directly install to SharePoint environment, which users can then enable them in their site.