Single-page applications, or SPAs, have transformed web development, ushering in an era where websites behave more like applications, enabling instantaneous interactions and a fluid user experience. Unlike traditional sites, which load each page separately, SPAs deliver content within a single document. This technique gives a website the feel of a desktop app, enhancing user satisfaction and making interactions faster and more intuitive. With SPAs, modern web development has embraced an architecture that serves both developers and users, optimizing speed, adaptability, and experience. In this article, we will explore the foundation of SPAs, compare them with multi-page applications (MPAs), analyze their pros and cons, and review some top frameworks and successful examples.