Schema Statistics are required to insure the proper plan for DML operations for a specific schema. They are used to determine whether indexes or full table scans are used and the type of joins to be used during execution. Schema Statistics are gathered by the DBMS_STATS.GATHER_SCHEMA_STATS procedure.
System Statistics are also necessary to determine appropriate execution plans. System Statistics measure the database server capabilities such as cpu, IO, and memory performance. System Statistics are gathered using the DBMS_STATS.GATHER_SYSTEM_STATS procedure.