Linux is a family of open source Unix-like operating systems based on the Linux kernel.
Linux is famous for giving life to old hardware,powering most servers around the world,being lightweight and FOSS (Free Open Source Software).

UNIX is a popular multi-user, multitasking operating system (OS) developed at Bell Labs in the early 1970s.

A kernel is the foundational layer of an operating system (OS).It has complete control over everything in the system.
Functions:

• Facilitates interactions between hardware and software components
• First program to load on startup
• Performs a system check and recognizes components like CPU,mouse etc
• Always resident in memory
• Run in the background and manage system resources

Distributions:
There are many types of linux distributions (distro or OS) but mainly the given four are most popular and many distributions are based on them:

1. Debian [FOSS and Stable with long term support] (Parent of Ubuntu,Kali linux)
2. Fedora [FOSS and Bleeding Edge]
3. Arch [Unstable and Bleeding Edge]
4. OpenSUSE [FOSS,Bleeding Edge and Developer focused]

Every distro has different package management system and architecture.

Desktop Environments:
There are also many types of Desktop Environments (DE) which decided how you interact with your distro and what system utilities are present. Basically it means how your distro and desktop look and feel. Most popular ones are:

1. Gnome (Modern looking)
2. KDE (Can look both old and new)
3. Cinnamon (look like Windows)
4. Xfce (Lightweight,default in Kali linux)

Linux Terminal
Most of the work in linux is done through it’s terminal (console).It is like cmd,powershell but more powerful. It is faster and more efficient as compare to GUI(Graphical User Interface).Most Linux distributions use bash shell by default.

Some of the functions possible through the Linux console terminal are:

• Extensive, system-wide configuration and administration
• File and folder administration
• The ability to access, transfer and share data between machines
• Extensive system monitoring
• Running various programs (both cli and GUI)

GNU Bash or simply Bash is a Unix shell and command (terminal) language. Bash is a command processor that typically runs in a text window where the user types commands that cause actions. Bash can also read and execute commands from a file, called a shell script.

Root User - It is a special user account with administration privileges like admin in windows.