Start with the goal, not the format
"I need an app" is a format request, not a goal. The real question is: are you explaining a business and capturing leads, running a workflow with logged-in users, needing on-device/offline capability, or connecting two sides of a transaction?
Website — explaining and capturing
If the goal is explaining your services and capturing inquiries, see Business Website Design — no login or persistent user data needed.
Web application — logged-in workflows
If users log in and take repeated actions against persistent data, see Website vs Web Application and Web Application Development.
Mobile app — device-native needs
If the goal specifically needs on-device hardware access, offline-first behavior, or app-store presence as a channel, see Mobile App Development — otherwise a web application or PWA (see PWA Development) usually serves the same goal for less cost.
Marketplace — connecting two sides
If the goal is connecting buyers and sellers (or two other distinct sides) with transactions between them, see Marketplace Development — a genuinely different architecture from the other three, not a bigger version of a web application.
Most businesses need one, not all four
It's rare for a first product to genuinely need all four — start with the one that matches your actual current goal, and let the others follow only if a real, separate need for them emerges.
Questions
Can I start with one and add the others later?+
Yes — this is the common, sensible path rather than committing to all four upfront.
What if I'm not sure which goal applies to me?+
Describe the actual workflow or problem you're solving — that determines the answer, not a checklist of buzzwords.
Is a mobile app always 'more serious' than a website?+
No — format doesn't equal seriousness; the right format is whichever matches your actual goal, not the one that sounds most impressive.