In specific, it is a column store, which is advantageous to do things like real time analytics over millions of data points via streaming market data. This has uses for HFT, but also for anyone who wants to do their own day trading.
VoltDB isn't a column store. It's designed for serializable OLTP workloads with really fast index updates, neither of which characterize column stores. You may be thinking of one of Stonebraker's other projects, Vertica.
