It's my understanding that Idea can do everything the other products can do. The others are just cheaper as they have limited features.

EDIT - From reading the comments I think appCode is separate but the others can be installed as plug-ins to IDEA, I presume for free.

so true, and IDEA is worthy of every penny you will spend.

IntelliJ IDEA has the functionality of all of their other products except appCode available as plugins, but the plugins aren't necessarily as up-to-date as the standalone products. The plugin version usually gets updated a couple months after a major version of the standalone product is released, in my experience.

For example, the plugin list for my copy of IntelliJ Ultimate 11 (cheap upgrade to 12 incoming, woo) shows a PHP plugin whose description says "PhpStorm/WebStorm 4.0.3 version". PhpStorm 5 was released in September, but a lot of the IntelliJ plugins for its features haven't seen an update since then.

Edit: Upgraded to IDEA 12, and it looks like the bundled plugins contain full WebStorm 5 functionality.

Y, IntelliJ IDEA does provide all the functionality that WebStorm provides + support for more languages.