Oracle is an RDBMS product. Specific releases of the product are known as Oracle9i, Oracle10g and Oracle 11g. Generally there are two releases within each major version. Questions tagged oracle10g are assumed to be specific to this version or features.
There are two releases of the Oracle 10g RDBMS software, 10gR1 and 10gR2. This tag also covers the current version of Oracle XE, the free release.
Questions tagged with oracle10g are assumed to be specific to this version or features introduced in this version. It is generally advisable to specify the full database version (e.g. 10.2.0.4) and it is often useful to include the underlying operating system.