Apps are software you download from the official marketplace. Use the tag when you are looking for one that will solve a specific problem.
Mostly one would install apps from the Store but apps can be sideloaded as well.
Where to get the apps
You can find the apps in the Store.
You can also sideboard apps if you are an enterprise or a developer.
Languages apps can be written in
Apps can be written using JavaScript/HTML, C#/XAML and C++.