Shodan, a search engine for internet-connected devices. They explain how Shodan gathers public information, like open ports and software versions, by "pulling banners" from millions of IP addresses, which is a legal form of passive reconnaissance. The tutorial demonstrates using Shodan's web interface and command-line tool to identify vulnerable systems, such as those running outdated software or exposing sensitive ports, for both ethical hacking (penetration testing) and defensive security purposes.