One of the cool new features of SharePoint 2013 is this concept of SharePoint Apps. Developers that are familiar with SharePoint 2010 will compare this with a Sandbox Solution. But that doesnt do it justice, Sandboxes are for children to play in, SharePoint Apps are for fully grown developers to develop business applications and sell them (licensing is starting to be defined). Just in the few MSDN articles that have been released about them I see stark differences.
For starters SharePoint Apps can cross site collections within the same "tenant" (a new SharePoint grouping that I will cover in a future article), this was a huge limitation of a sandbox solutions. Just think of what doors this opens up now, this means that all the HR site collections that are out in a farm can now talk amongst themselves if they are listed in the same tenant.
Another difference is the ability to have permissions, this means the business power user who downloads and activates an App from the App store can specify the exact permissions that app can have. No longer do you have to blindly give all 3rd party webparts complete control in your site collections. Here is an excerpt from MSDN about the app permissions (http://msdn.microsoft.com/en-us/library/office/apps/fp179922(v=office.15)):
Apps for SharePoint have permissions just as users and groups do. This enables an app to have a set of permissions that are different from the permissions of the user who is executing the app.
Unlike Sandbox solutions, Apps can now define their entire user interface. This means you are no longer limited to developing a webpart and hosting it inside a webpart page just to do some custom code. Think of how many times people make the comment "I want it to not look like SharePoint", well now you don't need to put your app in SharePoint and then brand SharePoint you can use an App hosted externally and get all your SharePoint lists, libraries, and context by default. Here are the UX options you have with SharePoint Apps, and here are some guidelines when styling your App: http://msdn.microsoft.com/en-us/library/jj220046(v=office.15)
There is so much more to come as I continue to fumble around in the 2013 beta preview, so stay tuned.