Client-side databases are storage systems that reside on a user's device, allowing applications to store and retrieve data locally. This setup enhances performance by reducing the need for constant communication with a server, making applications faster and more responsive. Common examples include IndexedDB and WebSQL, which are used in web applications to manage data directly in the browser. These databases are particularly useful for offline functionality, enabling users to access and manipulate data without an internet connection. They can also help improve user experience by allowing for quick data access and reducing server load, making them a valuable tool for developers.