There aren't minor versions for the most recent versions of Java. The major versions are Java 5 and Java 6. There might be minor versions again in the future. In older versions, eg 1.4.2, the 1.4 was the major version and the 2 was the minor version.
The major and minor version numbers of a java class are used to determine whether an implementation of a Java Virtual Machine can support a given class file. See section 4 of the JVM specification for details.