Web security involves protecting websites and web applications from various cyber threats. The document outlines the top 10 PHP application vulnerabilities in 2016, including information leakage, man-in-the-middle attacks, injection attacks, and SQL truncation exploits. It provides tips for preventing vulnerabilities such as checking error logs, updating software regularly, and using strong password hashing. The key is to stay vigilant by monitoring logs and source code for signs of intrusion and preparing to reinstall systems if needed.