All versions of Oracle. Also add a version-specific tag like oracle-11g-r2 if that context is important in the question.
Oracle is a relational DBMS (Database Management System) created by Oracle Corporation. It supports a large number of languages and application development frameworks. Its primary languages are SQL, PL/SQL and Java. When asking a question with the oracle tag, please be sure to include the full version number, (i.e., 10.2.0.5, 184.108.40.206), in addition the relevant version-related tags described below.
Oracle makes plethora of documentation available at http://docs.oracle.com.
Questions applicable to other versions should be tagged as noted above.