Zend Optimizer is a software tool, that's needed to run files protected with Zend Guard - a well-known application which is employed to encrypt PHP 4 and PHP 5 files with the objective to protect them from tampering with the code or reverse engineering. Zend Guard is used by a lot of organizations which create paid script apps, so if you get such an app for your site, you will most likely need Zend Optimizer to be present on the server where you will host it. If you are a programmer, you can use Zend Guard to protect your code and make sure that your site visitors or customers will be unable to alter it in any way. Websites that use Zend Optimizer generally perform much better due to the fact that their PHP code is precompiled, therefore it is already optimized and will be executed a lot faster.