A partition is a physically distinct store for a subset of the entries contained within a DSA (Directory Server/Service Agent A.K.A the LDAP server). The entries of a partition all share the same suffix which is the distinguished name of the namingContext from which the stored entries in the partition are hung from the DIT. A partition can be implemented using any storage mechanism or can even be backed in memory. The default storage mechanism for a partition is JDBM. The addition of such a partition is described in the Basic User's Guide. A partition with a different storage mechanism simply has to implement the Partition interface and by doing so can be mounted in the server at it's suffix/namingContext (described here).
The server can have any number of partitions (with any implementation) attached to various namingContexts which are published by the RootDSE (empty string dn "") using the namingContexts operational attribute. So if you want to see the partitions served by the server you can query the RootDSE for this information.