Oracle 10g is just the latest release of the Oracle database server, the successor to Oracle 9i and Oracle 8. The g in the name is just a marketing gimmick, like the i in 9i.
9i was ported to OS X some time ago, and now the 10g port is coming out. The g in the name is supposed to bring to mind the 'grid' notion that Oracle is promoting, where you build a database cluster from a bunch of smaller systems rather than a few larger ones. The Mac G5 servers might provide an attractive platform for doing that.