Is DBCA in Oracle for a single database or the entire system? I ask this because I was going to create a simple phone list database for learning.  But when I used dbca, I was asked did I want to install the sample databases, HR, etc.  It also had the default locations for all archive_log files, passwords for sys, sysman...I thought dbca was just to create a single database.

What is dbca for?
DBCA will be reside on Oracle home. You can create as many number of databases in system using dbca. It will create default locations based on your unique SID (and dbunique name).