VoltDB is a NewSQL database: it looks and feels like a relational database, and it provides the ACID compliance and immediate consistency that you would expect from a relational database, but it has a different underlying architecture.
VoltDB uses a shared-nothing architecture to achieve database parallelism, with both data and processing distributed across all the CPU cores within the servers composing a VoltDB cluster. In other words, the product should scale as the number of cores increases, not just by the number of servers. Horizontal partitioning also enables scalability. SQL access is supported via pre-compiled Java stored procedures. The unit of transaction is the stored procedure, which is Java interspersed with SQL.