This is explained (cough) in Java SE Naming and Versions:
The current release is Java Platform,
Standard Edition 6 (Java SE 6). The
previous release was Java 2 Platform,
Standard Edition 5.0 (J2SE 5.0).
Sun Microsystems simplified the
platform name in 2006 to better
reflect the level of maturity,
stability, scalability, and security
built into the Java platform. Sun
dropped the "2" from the name and
deleted the "dot number" (the number
following the period). Any updates to
Java platforms will simply be noted as
updates rather than adding a "dot
number" at the end of the platform
name.
Due to significant popularity within
the Java developer community, the
development kit has reverted back to
the name "JDK" from "Java 2 SDK" (or
"J2SDK"). The runtime environment has
reverted back to "JRE" from "J2RE."
(Note that "JDK" stands for "J2SE
Development Kit" in version 5.0.)
For more information on platform names
and version numbers, see the following
pages:
And quoting Java SE 6 Platform Name and Version Numbers:
At this release, the platform name has changed from J2SETM to JavaTM SE. The official name is JavaTM Platform, Standard Edition 6.
Both version numbers (1.6.0 and 6) are used to identify this release of the Java Platform. Version 6 is the product version, while 1.6.0 is the developer version. The number 6 is used to reflect the evolving level of maturity, stability, scalability and security of Java SE.
And J2SE Version 1.5.0 or 5.0?:
Both version numbers "1.5.0" and "5.0" are used to identify this release of the Java 2 Platform Standard Edition. Version "5.0" is the product version, while "1.5.0" is the developer version. The number "5.0" is used to better reflect the level of maturity, stability, scalability and security of the J2SE.
The number "5.0" was arrived at by dropping the leading "1." from "1.5.0". Where you might have expected to see 1.5.0, it is now 5.0 (and where it was 1.5, it is now 5).
So, to summarize, the whole naming is a mess but this is finally over and the current platform name is JavaTM Platform, Standard Edition 6 (abbrev. JavaTM SE 6). Two Products are delivered under the platform: the JDKTM 6 and the JRETM 6.
OpenJDK 6 is an open-source implementation of the Java SE 6 Platform, it's different from Sun's JDK.