Everything that exists on your phone is an app. For the average user, apps are just icons a screen that do exactly what they were made for. From a developer standpoint, an app is millions of lines of code which consists of a complex logical breakdown for a simple requirement & execution of said requirement to give the required result.
An app that appears simple may not be easy to code and vice versa. App development for smartphones has evolved over the years. It is still a new field which has been around for a measly 2 decades. App development has although evolved exponentially over the years and it has been parallel to the smartphone boom. App development will continue chugging as long as the smartphone industry progresses.
infographic by: www.appypie.com