What is memcache and memcached?

Question

PHP

Explain the difference between memcache and memcached for php?
And I installed them for php (php5.6-memcached), but it turned out that I still need to install memcached itself, which is now spinning as an independent process.
Do I understand correctly that this is a separate software, not a library for php, and does it need a php extension to work with it?

Answer 1

Yes, that's right:
memcached = a separate software
php-memcache = a library for php that allows you to work with memcached
in the same way working with mysql is arranged:
mysql = a separate software
php-mysqli = a library for working with it