A role is a collection of one or more privileges; it is a way of granting privileges to groups of people without having to assign the individual privileges everytime e.g. you could create a role called "marketing" & assign all the privileges that a marketing person would need, then when you create an account for someone who works in the Marketing Dept you only have to allocate the role to his account.
The role "DBA" is made up of the SYSTEM privileges, but with the ADMIN option included. The SYSDBA privilege allows whoever it is assigned to to do all the 'top level' things (such as create a database).
90% of users' problems can be resolved by punching them - the other 10% by switching off their PCs.