package-info.java
: “This file is new in JDK 5.0, and is preferred over package.html.”—javadoc – The Java API Documentation Generator
Addendum: The big difference seems to be package annotations. There’s a little more in the way of rationale in 7.4 Package Declarations.
Addendum: The annotation feature is also mentioned here and in Javadoc tip: Prefer package-info to package.html, cited here.
Addendum: See also What’s package-info.java
for?.