STS is a part of Oracle 10g's SQL Tuning Advisor feature. Each tuning set contains one or more SQL statements, plus the context information needed to interpret them correctly. A tuning set is used as input to the SQL Tuning Advisor, which examines the statements and makes recommendations for improving them.
SQL Tuning Sets (STS) capture the workload of an Oracle instance. Starting in Oracle 10g Release 2, these tuning sets can be transported from one instance to another to facilitate testing.