COST
Cost of the operation as estimated by the optimizer's cost-based
 approach. For statements that use the rule-based approach, this 
column is null. Cost is not determined for table access 
operations. The value of this column does not have any particular
 unit of measurement; it is merely a weighted value used to 
compare costs of execution plans. The value of this column is a 
function of the CPU_COST and IO_COST columns.

CARDINALITY
Estimate by the cost-based approach of the number of rows accessed by the operation.