Difference between LEGO Castle and LEGO Kingdoms
LEGO Kingdoms is a sub-theme of LEGO Castle. LEGO Castle is one of the oldest LEGO themes, and through the years it had various sub-themes like Knight's Kingdom, Fantasy Era, Dark Forest, etc. 

The current LEGO Castle theme is simply called LEGO Castle without a sub-theme name and I think that is causing the confusion. The previous LEGO Castle theme released between 2010 and 2012 was called LEGO Kingdoms. The only set that is still available from LEGO Kingdoms is the #10223 LEGO Kingdoms Joust set.

At the online LEGO Shop you will see that the two themes are listed separately. The reason being that they are actually two sub-themes of LEGO Castle, even though the current one doesn't have an official sub-name. So to be clear, LEGO Castle is the theme, with two current sub-themes; LEGO Castle and LEGO Kingdoms.

LEGO has always listed sub-themes separately. For example when LEGO Kingdoms first came out there were still some LEGO Fantasy Era sets left from the previous years, and although they were both sub-themes of LEGO Castle they were kept in separate categories. The same thing is happening now, except this time one of the sub-themes have the same name as the main theme. I hope this is not too fonfusing...(c;