Dynacache and Memcached are caching technologies for Java applications. Dynacache is built into WebSphere Application Server and caches content in JVM memory, allowing cache operations through POJO calls. Memcached is an open-source, high-performance distributed memory caching system where keys and values are transmitted over TCP/IP and cached content is not stored in JVM memory. Each technology has advantages such as Dynacache supporting memory and disk storage with size restrictions and faster operations through POJO calls without serialization, while Memcached does not consume memory in the Java heap.