Here is a quick backgrounder in naming conventions (for those unfamiliar), which explains the frustration by the question asker: For many *nix applications, the piece that does the backend work is called a “daemon” (think “service” in Windows-land), while the interface or client application is what you use to control or access the daemon. The daemon is most often named the same as the client, with the letter “d” appended to it. For example “imap” would be a client that connects to the “imapd” daemon.
This naming convention is clearly being adhered to by memcache when you read the introduction to the memcache module (notice the distinction between memcache and memcached in this excerpt):
Memcache module provides handy procedural and object oriented interface to memcached, highly effective caching daemon, which was especially designed to decrease database load in dynamic web applications.
The Memcache module also provides a session handler (memcache).
More information about memcached can be found at » http://www.danga.com/memcached/.
The frustration here is caused by the author of the PHP extension which was badly named memcached, since it shares the same name as the actual daemon called memcached. Notice also that in the introduction to memcached (the php module), it makes mention of libmemcached, which is the shared library (or API) that is used by the module to access the memcached daemon:
memcached is a high-performance, distributed memory object caching system, generic in nature, but intended for use in speeding up dynamic web applications by alleviating database load.
This extension uses libmemcached library to provide API for communicating with memcached servers. It also provides a session handler (memcached).
Information about libmemcached can be found at » http://tangent.org/552/libmemcached.html.
In summary, both are functionally the same, but they simply have different authors, and the one is simply named more appropriately than the other.
原文：http://serverfault.com/questions/63383/memcache-vs-memcached