Like mobile websites, mobile web apps are accessed via the browser, but they also have some important differences. For a mobile website, each time a new page is accessed, the device will download all of the page contents (including HTML, CSS, JavaScript, and data). For a web app, the majority of the download occurs on the initial visit and is stored locally. After this, only data is accessed from the web. This may sound like techno-babble, but it has some important real world impacts.
At a high level, web apps sit between native apps and mobile websites in terms of key factors such as reach and experience. Overall, web apps provide good experiences across multiple platforms, but still do not meet the full richness of current native solutions in demanding environments.