From the Zend Optimizer+ / Zend OPcache source README:
The Zend OPcache provides faster PHP execution through opcode caching and optimization. It improves PHP performance by storing precompiled script bytecode in the shared memory. This eliminates the stages of reading code from the disk and compiling it on future access. In addition, it applies a few bytecode optimization patterns that make code execution faster.
From the Zend Guard download page:
Zend Guard Loader is a free application that runs the files encoded using Zend Guard and enhances the overall performance of your PHP applications.
Zend Guard Loader only works with PHP 5.3 and 5.4. Prior to PHP 5.3 the runtime decoding was a part of the Zend Optimizer. Starting with PHP 5.3 the decoding capabilities were split out of the Optimizer and into the Guard Loader.
Zend Guard does not appear to currently support PHP 5.5.