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.
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.
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.
The functionality of apps and responsive websites have overlapping functionality such as one-click calling, social sharing, e-commerce, and navigation. However, apps require a download, while responsive websites are available to all and will allow users to connect with your business from anywhere, without additional marketing effort and expenses needed to point them to your app.
Responsive websites are also easier to maintain than apps. When you need to update a piece of information, all you have to do is change it in your website content management system and it will automatically make the changes across all device types, regardless of the operating system.