The alternative PHP cache (APC) is an opcode cache for PHP that provides a persistent application data store cache to PHP applications. This plugin allows the use of that APC data store as a Moodle cache store. In turn allowing you to use APC within Moodle.
XCache is a PHP opcode cacher designed to accelerate the performance of PHP execution. One of the features it provides is a variable data store that can be used within PHP code to persist variables between requests. This plugin is an XCache cache store designed to utilise that variable data store and allow Moodle to make better use of XCache.