Hi Sanjay,
Are you looking for something like select ... from ( partition xx of table ) or from table where partition = xx, then as Lars mentioned it might be undocumented.
But if you execute a query something like select ... from table where partition_criteria_column = some_value, then the data will be selected from the required partition.
I hope you had read the Table Partitioning section in the Admin Guide.
It mentions about Partition pruning as follows:
Partition pruning
Queries are analyzed to determine whether or not they match the given partitioning specification of a table. If a match is found, it is possible to determine the actual partitions that hold the data being queried. Using this method, the overall load on the system can be reduced, thus improving the response time. For example, if a table is partitioned by year, a query restricted to the data of one year is executed only on the partition with data for this year.
I think the Admin guide will clear most of your doubts on partitioning.
Regards,
Ravi