Linux Basics
Users
Root
root is the name of the system administrator account.
Root has user id 0 and nominally unlimited privileges.
Exceptions:
In modern UNIX systems, SELinux or Apple's SIP (aka rootless system), even root is limited in what it can do.
NOTE: For the rest, read this answer: https://unix.stackexchange.com/a/291467
Groups
File System Heirarchy
/
/boot
/boot/efi
/home
/bin
/sbin
/lib
/proc
Process information virtual file system
/dev
System devices virtual files
/dev/ram - Ramdisk device
/dev/tty
/dev/null
/dev/zero
/dev/initrd - Bootloader initialized ramdisk
/dev/loop
/dev/mouse
/dev/random
/dev/rtc
etc.
/sys
/swap
/root
/usr
/usr/share
/usr/bin
/usr/lib
/usr/include
/usr/local
/usr/src
/opt
Manually installed binaries