APC (Alternative PHP Cache) is open source opcode caching mechanism.
I will not talk about which one is better, XCache or APC, i found out that some of them work little bit different in different environments and with different scripts. And this is probably why you did not came here to read about. We will see how to speed up some work.
Personally, i don’t like APC much but my clients do, so i needed to find out some new ways of optimizing caching to work on high load servers which also include memcached and other boys and girls.