Database Versus System Statistics

In the article regarding gathering CBO Statistics, it states: QUOTE When an Oracle database is created, a job will be scheduled that will generate the database statistics for you. You will still need to collect system statistics however, as these are not collected by the automatic statistics gathering mechanism.

what is the difference between "database statistics" and "system statistics"? In other words, do I need to run this script for each schema owner in my 10g/11g instance?

variable whoami varchar2(20);
begin
select user into :whoami from dual;
end;
exec dbms_stats.gather_schema_stats( -
ownname => :whoami, -
options => 'GATHER AUTO', -
estimate_percent => 15, -
cascade => true).