Apache is a free and open-source cross-platform web server software.
CentOS is a Linux distribution that provides a free, community-supported computing platform functionally compatible with its upstream source, Red Hat Enterprise Linux (RHEL).
HTTP Strict Transport Security (HSTS) informs browsers that the site should only be accessed using HTTPS.
Twitter is a 'microblogging' system that allows you to send and receive short posts called tweets.