Very good post by Richard Niemiec (thank you)
A partitioned index in Oracle 11g is simply an index broken into multiple pieces. By breaking an index into multiple physical pieces, you are accessing much smaller pieces (faster), and you may separate the pieces onto different disk drives (reducing I/O contention). Both b-tree and bitmap indexes can be partitioned. Hash indexes cannot be partitioned. Partitioning can work several different ways. The tables can be partitioned and the indexes are not partitioned; the table is not partitioned but the index is; or both the table and index are partitioned. Either way, the cost-based optimizer must be used. Partitioning adds many possibilities to help improve performance and increase maintainability.
Read on here