Social engineering is an art of manipulating human behavior in order to bypass the security controls of the target system. Let's take the example of an organization, which follows very stringent security practices. All the systems are hardened and patched. The latest security software is deployed. Technically, it's very difficult for an attacker to find and exploit any vulnerability. However, the attacker somehow manages to befriend the network administrator of that organization and then tricks him to reveal the admin credentials. This is a classic example where humans are always the weakest link in the security chain.
Kali Linux, by default, has a powerful social engineering tool, which seamlessly integrates with Metasploit to launch targeted attacks. In Kali Linux, the Social-Engineering Toolkit is located under ;Exploitation Tools...