People use these three terms loosely, and the confusion costs real money. Someone asks for an app when they need a website, or builds a website when the idea only works as an app. Before you spend anything, it helps to know which one your idea actually is, because the wrong choice means paying for the wrong thing.
A website is for showing information and capturing interest. If your goal is to be found, explain what you do, and get enquiries or orders, a website is almost always the answer, and usually the cheapest, fastest route to results. Most businesses asking for an app actually need a really good website first.
When you cross into app territory
A web app is software that runs in the browser, where users log in and do things: a dashboard, a booking system, a tool, a SaaS product. You reach for a web app when people need to perform tasks and work with data, not just read pages. It is a product, not a brochure, and it is built and priced differently.
A mobile app lives on the phone and earns its place when you genuinely need what only a phone offers: push notifications, offline use, the camera, or a presence on the home screen people open daily. Apps cost more to build and maintain, so the honest question is whether your idea truly needs the phone, or whether a fast mobile website would do the same job for far less.
The trap is jumping to a mobile app because it sounds impressive, when a website or web app would serve users better and cost a fraction as much. Start from what your users need to do, and the right choice usually picks itself. If you are weighing it up, tell me the idea and I will give you a straight answer, because the cheapest mistake is the one you avoid before building.