Oracle has a wide product range; business-database,
middleware, business intelligence, business applications, collaboration etc.
but companies’ primary expertice is with their database product -> http://otn.oracle.com/database
I - As a user I choose Oracle because
A- This community has no support or information problem; tahiti.oracle.com metalink.oracle.com asktom.oracle.com otn.oracle.com blogs.oracle.com and forums.oracle.com are great places information is shared, with high quality. Community leaders like Thomas Kyte, Jonathan Lewis and Steven Feuerstein always guide you with their experiences available through their books, blogs and articles.
B- There are lots of supplied packages within database; nearly eveything is thought and solved simply for the developers with these packages; 'DBMS%' 'UTL_%' 'OWA_%' 'SDO_%' 'HTMLDB_%' 'CTX_%' etc. With them you will be more time to market and they have no maintainance or support problem, really reliable applications -> http://download-east.oracle.com/docs/cd/B19306_01/appdev.102/b14258/toc.htm
C- As Mr. Kyte says; "all databases are fundamentally different and, when designing your application, you must approach each as if you never used a database before. Things you would do in one database are either not necessary, or simply won’t work in another database."
Oracle with its undo mechanism never blocks readers and locking is row level, this makes Oracle a high concurrent database system. Where as a simple update query does a lot behind and in this system your primary approach must be reducing logical I/O. In this high-concurrent system readers do not block writers and writers do not block readers.
Oracle allows you a high degree of read consistency, if you start a long running query you do not see the changes of the other users until your query ends. Also the system protects you from phantom-uncommited data changes. This is also handled with undo mechanism.
In Oracle you will learn that concurrency and consistency can be achieved very fast and correct, every time -> http://download-uk.oracle.com/docs/cd/B19306_01/server.102/b14220/consist.htm#i13945
D- As Mr. Lewis says: "Oracle is not just a bucket into which you can throw your data, even for a basic set of documentation you will find ~10,000 pages of documentation you will be reading. The error messages alone ~1,000 page." This is a complex system with lots of beautifull features, just like an operating system.
E- Oracle has a high degree of recoverability, Oracle will recover itself automatically to the moment of the contingeny and your data will be in a consistent state; any incomplete transaction will be rollbacked and commited data will be available for you after you startup your instance.
F- PL/SQL is the native language of Oracle, for highly scalable applications needs PL/SQL and Oracle's SQL extentions like analytic functions are great when you are working on huge amount of data -> http://download-uk.oracle.com/docs/cd/B19306_01/appdev.102/b14261/toc.htm
Oracle may not be cheap for some companies but after reading above references if you will be doing a mission critical database operation, I believe you will eventually choose Oracle without hesitation. Because these are the reasons why Oracle Database is #1 with 48.6% share in the market at 2005 -> http://www.gartner.com/press_releases/asset_152619_11.html
II - 12 important links for Oracle newbies
1. History of Oracle
2. Oracle Product Family and Pricing Highlights
3. Oracle Database and Corporation From Wikipedia, the free encyclopedia
4. Oracle® Database Concepts 10g Release 2 (10.2)
5. Oracle database official documentation
6. Free Database 10g Release 2 Express Edition for Windows and Linux
7. Free Development Environments for Oracle Database
8. Oracle Technology Network(OTN) portal for downloads, articles and sample code
9. Oracle Technology Network(OTN) forums
10. AskTom forum by Thomas Kyte
11. Free Oracle Magazine
12. Turkish Oracle Users Group