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.
Oracle makes plethora of documentation available at tahiti.oracle.com.
Questions about SQL should be tagged sql. If the question applies to an Oracle database it should probably also be tagged oracle.
Questions about PL/SQL should be tagged plsql as well as oracle.
Questions specific to a particular database version may be tagged with a version tag such as oracle11g-r2, oracle-11g, oracle-10g or oracle-9i, but they should also be tagged oracle.
Questions applicable to 220.127.116.11 through 18.104.22.168 should be tagged both oracle-11g and oracle.
Questions applicable to 22.214.171.124 through 126.96.36.199 should be tagged both oracle-11g-r2 and oracle.
Questions applicable to other versions should be tagged as oracle-10g, oracle-9i or oracle-8i as well as oracle.