Smarty is a templating engine written in PHP. Smarty can be useful for developing websites as it seperates your presentation code (ie, HTML/CSS) from your business logic (ie, PHP). Seperating your presentation and logic is important on large projects when you have designers that can't code and coders that can't design. It also keeps your code clean and easy to maintain. Other reasons why to use it are here: http://smarty.php.net/whyuse.php

Smarty is a fairly simple language, yet very flexible and easy to extend. The downside to smarty is the amount of code it uses to work it's magic, however some of the parsing overhead can be mitigated with Smarty's built-in caching scheme and a PHP compiler cache such as APC, !ZendOptimiser?, Turck MMCache, Afterburner or PHP Accelerator.

Smarty can be found at http://smarty.php.net/