Since this comes up every time there's a Jetbrains sale, here's the rundown on how the products relate to each other.

First, the easy ones. ReSharper, dotTrace and dotCover are all independent products that don't really overlap with the ones I'm about to talk about.

The "Core" doesn't exist as a product, but it's all the things that are product-independent and aren't provided by plugins like the editor, HTML/CSS/JS editing support, debugger, theming, etc. I'm mentioning it because each IDE ends up with a slightly different version of the Core based on when it was released, which means that using IntelliJ with the Python plugin might not have all the features of PyCharm if PyCharm got a major version update more recently than IntelliJ.

WebStorm (JS/HTML/CSS IDE) has the core functionality plus Node.js plugins. You can install other plugins, but only a limited subset.

AppCode (Obj-C IDE) has the core functionality plus product-specific Obj-C plugins that you can't install in any of the other IDEs. AppCode only shares a few plugins with other IDEs, like the Vim emulation.

RubyMine, PhpStorm and PyCharm have the core functionality plus language/platform-specific plugins for Ruby, PHP and Python respectively. You can install other plugins, but nothing that would turn one of these into the other, for example installing the PHP plugin in PyCharm.

IntelliJ IDEA has the core functionality plus a huge variety of Java plugins. It can also use the language-specific plugins from WebStorm, RubyMine, PhpStorm and PyCharm as well as others that are downloadable. It can install almost all of the plugins that are supported on the platform - the only exception I'm aware of is Obj-C support.

Hope that answers some questions.