Depending on the options you have installed, default users varies, but you might find these users:
SYS, SYSTEM, OUTLN, DBSNMP, MTSSYS, AURORA$ORB$UNAUTHENTICATED, SCOTT
DEMO, ORDSYS, ORDPLUGINS, MDSYS, CTXSYS

SYS is the Oracle dictionary owner, whereas SYSTEM is a user who has been granted all DBA roles and has access to entire dictionary. In other words you can say SYS is the owner of the company and SYSTEM is the manager who has access to all company's funds (and who can screw the company's financial status if he wants)