This guide explains how to install memcached and the PHP5 memcache module on a Debian Etch system with Apache2. memcached is a daemon that can store objects in the system's memory (e.g. results of database queries) which can speed up your web site tremendously.
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You can use memcached over a network (i.e., install your web application on one server and memcached on another server), but usually you install both on one server to avoid the networking overhead.