Progressive Web Apps combine web and native app capabilities, delivering app-like experiences through browser technologies. Built on service workers, web app manifests, and HTTPS, PWAs work offline, install to home screens, and send push notifications — all without app store gatekeeping. The critical insight: PWAs don't replace websites or native apps; they bridge the gap by giving web content the reach of URLs and the engagement of apps, with 68% lower development costs than maintaining separate iOS and Android codebases.