There is really no difference, but today, "apps" are seen as something you buy and/or download from an online store that will work on multiple devices. "I'm going to download that app.. it costs $1.99 and will work on my tablet and phone." This has become a popular term because of the rise of tablets and smartphones. For example, "Modern UI" apps are programmed in Visual Studio in C#, have cross-compatibility between devices, and usually incorporate web scripting elements.