Oracle licensing is very well explained in the following link
Oracle licenses the database servers by Oracle Database Editions. In 11g there are 5 editions with Oracle Enterprise Edition being the most inclusive one.
Other licenses relevant to business are Standard Edition One (entry level) and Standard Edition.
On the top of the ‘Edition’ licenses there are ‘Extra Cost Options’.
Enterprise Edition, for example comes with Data Guard included, but Active DataGuard is an ‘Extra Cost Option’. Partitioning is always ‘Extra Cost Option’.
On the subject of the DataGuard, experienced DBA can set up standby databases on Standard Edition without using DataGuard, and thus provide necessary redundancy to the system. DataGuard is in a way an automated management on the top of the basic functionality of the standby databases. It is very slick, but at a price.
Other useful linkes on licensing :