It`s a jit-compiler for PHP. Code is compiled once to an intermediate representation (like Java) which makes execution faster on subsequent request.

Code does not have to be written in a certain way most of the time because hhvm is 98% compatible to all major PHP-Frameworks[1]. It also features a built-in webserver alltough static performance is lower than nginx.

[1] http://www.hhvm.com/blog/2813/we-are-the-98-5-and-the-16