When a client tells us “I want a mobile app”, they often mean very different things. Sometimes they want a genuine native app to download from the App Store. Sometimes they simply want their website to work well on a phone. These aren't the same thing, not technically, not in terms of cost, and not in terms of use.
A mobile site: your website viewed from a phone
A mobile site is simply your website displayed on a smartphone screen. If it's well built, the layout adapts automatically to the screen size, this is what we call responsive design. Users access it from their browser (Chrome, Safari), without installing anything. It's the simplest and least expensive way to have a mobile presence.
Its limitations: it needs an internet connection, it can't access the device's advanced features, and it can't send push notifications.
A mobile app: software installed on the device
A native mobile app (iOS or Android) is software that users download from a store and install on their phone. It can work without a connection, access the camera, GPS and contacts, send notifications, and offer a smoother, faster experience than a website.
The trade-off: it costs a great deal more to develop, has to be published and maintained on the stores, and asks your users for the extra commitment of installing it.
So where does the PWA fit in?
A Progressive Web App (PWA) is the middle ground: it's a website that behaves like an app. It can be installed on the home screen, work partly offline, and send notifications. The development cost is close to that of a website, and the user experience is close to that of an app. For many projects, it's the best option.
The question isn't “site or app?”, it's “which behaviour is essential for my users, and which solution delivers it at the best value for money?”
Understanding this distinction lets you make an informed decision before you invest. If your goal is to be accessible on mobile, a good responsive site or a PWA is enough in most cases. If you need native features or a very frequent, intensive experience, a native app is worth considering.