The Oracle Optimizer: Explain the Explain Plan (White Paper)

Oracle White Paper
The Oracle Optimizer Explain the Explain Plan

Introduction
The purpose of the Oracle Optimizer is to determine the most efficient execution plan for your queries.
It makes these decisions based on the statistical information it has about your data and by leveraging Oracle database features such as hash joins, parallel query, partitioning, etc. Still it is expected that the optimizer will generate sub-optimal plans for some SQL statements now and then. In cases where there is an alternative plan that performed better than the plan generated by the optimizer, the first step in diagnosing why the Optimizer picked the sub-optimal plan is to visually inspect both of the execution plans.

Examining the different aspects of an execution plan, from selectivity to parallel execution and understanding what information you should be gleaming from the plan can be overwhelming even for the most experienced DBA. This paper offers a detailed explanation about each aspect of the execution plan and an insight into what caused the CBO to make the decision it did. 

http://www.oracle.com/technetwork/database/bi-datawarehousing/twp-explain-the-explain-plan-052011-393674.pdf