Cloudflare is a web-infrastructure and website-security company, providing content-delivery-network services, DDoS mitigation, Internet security, and distributed domain-name-server services.
HTTP Strict Transport Security (HSTS) informs browsers that the site should only be accessed using HTTPS.
Linkedin Sign-In is an authentication system that reduces the burden of login for users, by enabling them to sign in with their Linkedin account.
Twitter is a 'microblogging' system that allows you to send and receive short posts called tweets.