How are the DBA, SYSDBA, SYSOPER roles and SYS, SYSTEM users related in Oracle?

I don't know if I correctly understand the privileges of these built-in roles and users, but this is a list of the information that I have found: * The DBA and SYSDBA roles enable you to do everything. If this is true, then why are there two names for the same thing? * The SYSOPER role enables you to do everything except accessing users' data; * The SYS user has the DBA and SYSDBA roles, so they can do anything; * The SYSTEM user has the DBA role, so it seems that they could do anything, but I have read that they cannot do backups, recovery, and DB upgrade. Is my understanding correct?