[AgentEpsilon]

Haxe is basically several different things.

A language: It's language was originally designed to be a successor to ActionScript 3, as Haxe was originally designed to replace Flash.

A cross-compiler: The most interesting part in my opinion. Basically you can write Haxe code and target many, many different platforms. Mostly how this works is by transpiling, or translating the code into valid code in other languages.

A cross-platform standard library: This just means that it includes its standard library whenever it cross-compiles.

Ways to access native capabilities: Depending on your desired compilation target, you can use specific functionality only available on that target.

[IanCal]

I've used it before as a demo building something to run on flash based set-top boxes. Hey couldn't process XML quickly so I moved the code that did it to run on a server and use the built in serialise/deserialise functions. Then I just compiled one side to flash and one side to PHP.