Everything on SharePoint 2013 is an app. All the things you might be familiar with (list, libraries, code) are packaged into an app for SharePoint. This way, whenever the user wants something for the site, there’s an app for that. Where do you get apps? From the store. For Office365 there is a public store that contains public apps. Some are free, some have other licensing models. You can also get apps from a private app catalog for your SharePoint site.
For more detailed technical information on this topic, see Brian Moore’s blog post at http://blogs.msdn.com/b/lightswitch/archive/2013/04/29/publishing-lightswitch-apps-for-sharepoint-to-the-catalog.aspx