The javax classes are java eXtension classes. Generally this means they were not (or are still not) included in a JDK release but were a separate library. For example, the original release of JDK 1.0 didn't include Swing and it became available as a separate library, so it was javax.swing. Even once it became a part of the JDK, the naming convention stays (probably for backwards compatibility).
Core JDK classes/interfaces, java.util.List for example, which were first released with the JDK use the java package name.