Memcached is an open-source, high-performance caching system designed to speed up dynamic web applications by alleviating database load. It stores data in memory, allowing for quick retrieval of frequently accessed information, which reduces the time it takes to generate web pages. Developed in 2003, Memcached is often used in conjunction with various programming languages and frameworks, making it versatile for different applications. By caching data such as user sessions or database query results, it helps improve the overall performance and scalability of web services.