Memcached is the name of the Memcache daemon.
Using Memcache with PHP you are able to store PHP sessions in memory rather than in separate files on disk. This results in many times faster session management. This is especially useful when you've set up a load balancer and a few web app servers behind it but you plan to share sessions for each of your servers. You can run memcache on one of your servers or a separate server and make all your servers cache to the one running Memcache.
To get started:
sudo apt-get install memcached
Go through the config file in /etc/memcached/memcached.conf and see if the defaults are good for you.
Now install php's memcache client
sudo apt-get install php5-memcache
When asked, enable session handler support. Also, add
session.save_handler = memcache
session.save_path = "tcp://10.0.0.1:11211"
to your php.ini (probably in /etc/php5/apache2/php.ini). Change the ip to your memcache server address or to 127.0.0.1 if you run it locally.
Start the service with
Before installing anything, you really should read up on what the package does otherwise you will not be able to get the most out of it.