The general goal of a phone app and a mobile-friendly site is the same; make it easy for people to easily interact with your company with their smartphone.
What's an App? (Short answer)
A mobile app (short for application) is a program that is developed for your business but must be downloaded and installed onto a user's mobile device. This requires your customer to know about and actively seek out your app.
What's a Mobile-Friendly Site? (Short answer)
A mobile-friendly website is simply a website adapted to tablet and smartphone formats. A common name for this is 'responsive website' meaning that the layout of the website automatically responds to the size of the screen it is being displayed on so that is it most easily used on the given screen size.