It is a database for large scale website that is much better, more reliable, more features than mysql, but it costs an arm and a leg. The new version is advertised as 3 times faster than major competitors. For more information, go to oracle.com
Be warned that Oracle is NOT for the faint hearted. It is the world's most powerful database package, but if you are running a small business / website Oracle is certinaly not the right tool for you. It's incredibly expensive (licenses cost in excess of 100,000 UK pounds) and very hardware intensive - it also requires in depth knowledge to run properly. Oracle DBAs (Database Analysists) are professionals who's soul job is to run / develop Oracle database driven applications, and they get paid a lot of money for their knowledge.
Unless you are going to be running a database full of gigabytes of mission critical information that will be accessed several times a second, Oracle is probably not for you. mySQL, Sybase, postgresql are all free and much easier to use than Oracle - plus they are fine for most applications.
Oracle is the kind of thing places like Amazon or major credit card companies use.