This is one of the vulnerability which features in OWASP top 10 vulnerabilities. This vulnerability arises when web admins do not configure their website properly. For example, sometimes a web application might behave in a way that it is not supposed to and it reveals the buggy line in the code publicly which allows attackers to experiment on their site for other vulnerabilities.

A server is a normal system with a web server application like Apache running over it ,which makes a normal system serve other systems.

It is console that is provided by the operating system to let users interact with it deeply. Some examples of shell are bash ,command prompt ,brew and terminal.

It is a proxy server for your mail server that supports SMTP protocols and handles all Email transfers. In enterprises, SMTP Relay performs multi-functional tasks such as sending or receiving bulk mails.

As we all know, engineering is something which prepares a human to make maximum from minimum availability. Similarly, social engineering is what a human experiments on humans not machines. A person who tries to do social engineering is just using his socially available or informative assets to extract some information from him.