HyperDB is a very advanced database class that replaces a few of the WordPress built-in database functions. The main differences are:
* HyperDB can be connect to an arbitrary number of database servers,
* HyperDB inspects each query to determine the appropriate database.
It supports:
Read and write servers (replication)
Configurable priority for reading and writing
Local and remote datacenters
Private and public networks
Different tables on different databases/hosts
Smart post-write master reads
Failover for downed host
Advanced statistics for profiling
WordPress Multisite
It is based on the code currently used in production on WordPress.com with many MySQL servers spanning multiple datacenters.