You all must know that heart of every Linux distro is terminal (counterpart of CMD of Windows in Linux). It is much much powerful than its counterpart CMD in windows. You can master any distro by having a good grip over terminal. GUI is different for different distros but same UNIX terminal lies in core all of them.
For having a good grip over terminal you need to know all basic terminal commands. I am gonna tell you 20 most important commands of Linux.

(1) su
su stands for superuser. Linux has limited user access that is why it is considered and de facto is secure. It asks for administrative password at every gateway.
Switching user to administrator is a lot simple than windows.
Type su in terminal window and it’ll ask for password.

su

(2) mv
Its like cut and paste

cp ~$ mv /home/ashish/Desktop/1.png /home/ashish/Desktop/picsr/1.png

(3) rm

Its like delete in windows.

rm files

(4) gzip
Compresses the file and give it a extension .gzip or .gz. Winrar/Winzip do the same job over windows.

gzip filename

(5) cat
It displays the content of file right there.

cat [Options] [File]

(6) mount
It mounts the CD/DVD and other external devices.

mount | column -t

(7) pkill/pkillall
Its like pressing ctrl+alt+delete and forcefully terminating a humpy process. Actually its better than classic kill and killall which requires pid number.

pkill [-signal]

(8) passwd
Its for changing the password.

passwd [options...]

(9) halt
In case of emergency for avoiding loss of data forcefully shutting down.

halt [-d | -f | -h | -n | -i | -p | -w]

(10) reboot
Simply restart the system

reboot [-d | -f | -i | -n | -w]

(11) clear
It clears visible area of console.

clear

(12) vi
It is like notepad editor in Windows. It can be used inside terminal.

(13) man

It is used to read information, help and other details about command, information or process.

man commandname [options]

(14) gcc

It invokes C/C++ compiler.

(15) ifconfig
Its Linux counterpart for Windows’ ipconfig.

ifconfig [options]

(16) lock
It temporarily locks the terminal.

(17) pwd
It prints present working path.

pwd [options]

(18) cksum

Checksums and count the bytes in file specified. Prints CRC check-sum and byte counts of each FILE.

cksum filename

(19) gunzip

It un-compresses file.

gunzip [-Z]

(20) installpkg
It is used to install one of the packages from the program sets.

installpkg [options]