We remember. Years ago we called them Programs or Software. Since Apple launched the App Store we call them Apps, even on PC or other devices. Even the TV now has apps. Researchers forecasted the Web is dead, as its content and interaction move to App's of the provider instead. This is already happening as statistics prove (google for access numbers of mobile for Google, Youtube etc.)
Now Apple opened maps, messenger, Fotos and more of their apps to developers, allowing them to develop plugins. In my opinion Apple isn't doing this for fun, they are forced to do this.
The trend is less App switching. We have so many apps now that people simply no longer want to switch between them. Like going to maps to look for a restaurant, reserve in another app, book an Uber in their app to drive there.
This also explains the incredible value WeChat and Line have now. Check this out:
This means WhatsApp is under pressure. The big secret of those new chat programs in Asia is their plugins. You can do everything inside the chat you normally do in Apps. In fact in China WeChat is so massive that if you aren't in WeChat with your service you virtually don' exist.
Facebook looks into this direction with their Messenger, but WhatsApp seems to sleep regarding this matter.
We'll see. the world of apps is changing again.
How this effects mobile games? Well, the first games making millions are inside WeChat. The other way around works too: why isn't your chat and clan chat no in those chat apps instead doing your own? There is my hint.
(Update) Techrunch well written article on the Messaging War: