Hey guys... How do we classify apps on the basis

Question

Hey guys... How do we classify apps on the basis

of its complexity?
Like how do we know if the app will be simple or will it be complex?
Well... I can classify apps as simple and complex but I don't know on what basis am I able to do it?
For example, among 1. Alarm app, 2. Calender app, 3. Business record keeping app, 4. Property bidding app, 5. E-commerce app...
Among them I know first 3 are supposed to be called as simple apps and last 2 are complex... But why is it?