Service workers are proxies between the web page and network that provide cached versions of a site when no network connectivity is available, enabling offline-first applications and improving performance during slow or intermittent connections. They run on a separate thread in the browser and have a predetermined lifecycle with events such as install, activate, and fetch, which manage caching and updates to end users. To get started, developers can register a service worker by adding a few lines to their JavaScript file and deploy it to a supported browser like Chrome, Firefox, or Opera. However, there are limitations and potential gotchas, including the requirement for HTTPS, risk of man-in-the-middle attacks, and the need for cache invalidation strategies.