In this day and age its easy to take the "simplicity" of the modern relational database for granted. When you first learned about relational databases you might have said to yourself "this is just a bunch of spreadsheets!", and more or less you'd be right. However, this wasn't always the case, it was Dr. E.F. Codd, an IBM researcher, that first developed the relational data model in 1970. In 1985, he published a list of 12 rules known as "Codd's 12 Rules" that defined how a true RDBMS should be evaluated. Understanding these rules will greatly improve your ability to understand RDBMS's in general, including Oracle. The following are Codd's rules1.1:
Understanding these rules provides you with a fundamental understanding of the relational database and how it is constructed. Of these rules the most difficult one to come to terms with is rule 4, stating that all information describing the database must itself be contained in the database, this is where system tables or data dictionaries come in and why its so difficult to start exploring the database without a basic understanding of the relation database design rules. Its not terribly diffrent than building a webserver that must be configured using a CGI, while it isn't intiative, it is an admirable design decision.
You by no means need to bother memorizing this list of rules but simply stashing them away in your long term memory will help to clarify some design decisions used by modern relational databases that might otherwise seem odd.