Git can be configured more than just name and email.

Is part of Public Key Infrastructure (PKI), a type of this is asymmetrical encryption. This is a mathematical trap-door function that enables secure encrypted communication within public-network, preventing eavesdropping.

Proxy and Reverse-Proxy are just networking terminology. It helps to visualize it as between client and server.

Secure SHell is an application implementation to securely communicate and give commands to remote host. It work at Application Layer.

Chatting in real-time invovled server-client communication. For example, PHP web is best for Create-Read-Update-Delete operation, but it is inefficient for chatting, as the client need to pool against the server for new data periodically. This would not scale for the server against the growth of user base.

Hadoop: A set of open source programs and procedures which anyone can use as the “backbone” of their big data operations.

Linux (also known as GNU/Linux) is a clone of UNIX, the multi-user operating system which can be accessed by many users simultaneously. Linux can also be used in mainframes and servers without any modifications (and this attracts attackers).