MPM prefork is a non-threaded pre-forking server. Is it thread safe.
MPM worker is a multi-process multi-threaded server. It is not thread safe.
Prefork is generally recommended for servers that will be serving PHP applications, as may libraries are not thread safe. As such, hosting providers often use prefork.
References
- http://httpd.apache.org/docs/2.2/mod/prefork.html
- http://httpd.apache.org/docs/2.2/mod/worker.html
- http://serverfault.com/questions/45042/which-to-install-apache-worker-or-prefork-what-are-the-dis-advantages-of-each
