As an SA you need to be able to divide the various files in Oracle into logical groups mentally. Knowing what goes where will help you get a better idea of how you can help maintain Oracle from a systems level. For this discussion, we'll assume that you have installed Oracle according the the OFA discussed earlier.
We can device the installation into 2 main catagories right off the bat: oracle distribution files and instance files. Oracle distribution files would be everything that you installed when you installed Oracle. Database files would be all the files that pertain to a given instance of the database.
It's important here to clarify the diffrence between a "database" and an "instance". A "database" is the collection of files that store your data and all the metadata and configuration information to govern how that data is accessed, utilized, etc. An "instance" is a group of proccesses that make your database accessable. So, the datafile "users01.dbf" is a componant of the database and the proccess logwritter proccess "ora_lgwr_SID" is a componant of the instance.