The distinction between a web app and a mobile app primarily lies in their design and usage. A web app operates through a web browser and is accessed via the internet, making it platform-independent. In contrast, a mobile app is specifically designed for use on mobile devices and is usually downloaded from an app store. Web apps often utilize HTML, CSS, and JavaScript, while mobile apps are typically developed using platform-specific languages. This difference affects user experience, accessibility, and functionality across various devices.