Why does the url of some websites start with 'https' and not 'http'?

HTTPS is a relatively secure communications protocol over Internet over HTTP (you see HTTPS means HTTP Secure). Many popular user-based services like Google, Facebook, Twitter have switched to HTTPS protocol as default over security issues. According to Wikipedia, as of September '13, 24.6% of Internet's 168088 most popular web sites have a secure implementation of HTTPS.