Greg Luck following up on his article Ehcache: Distributed Cache or NoSQL Store? talks about architectural differences between distributed caches, NoSQL database, and RDBMS and where distributed caches fit:
NoSQL and RDBMS are generally on disk. Disks are mechanical devices and exhibit large latencies due to seek time as the head moves to the right track and read or write times dependent on the RPM of the disk platter. NoSQL tends to optimise disk use, for example, by only appending to logs with the disk head in place and occasionally flushing to disk. By contrast, caches are principally in memory. […] With RDBMS a cache is added to avoid these scale out difficulties. For NoSQL, scale out is built-in, so the cache will get used when lower latencies are required.
Original title and link: Distributed Caches, NoSQL Databases, and RDBMS ( ©myNoSQL)