1) The best thing here is to have a look at the Oracle Concepts Guide. Here's the index page ... http://download-west.oracle.com/docs...b14220/toc.htm Have a look at the "Oracle Database Architecture" section and it will give you an extremely thorough grounding in many important concepts -- tablespaces, data files, tables, indexes, views etc.

2) Very difficult to answer. It could just be half a dozen important ones, oe if you include all the ad-hov desktop databases that individual employees throw together you could be looking at over a hundred.

3) Absolutely. Oracle is not just the behemoth of Enterprise Edition -- look at the Standard Edition and how you could use Oracle's Application Express to centralise a shed-load of minor (and quite major) applications.