Using a Linux distro is a completely different thing. If you are a windows user you can see they have a drive based structure for file system. If I take an example there is a system directory called C:/windows/system32 . If you have another partition they will be D:\ , E:\ etc. All of files and folders are inside of those drives. So every file path is starting with a drive letter.  But in Linux we treat all thing as directories. The directory tree is begin with '/' . We call it root of the tree. 
thilan@bt:~$ ls /
0     dev   initrd.img      lib64       mnt   root  snap  tmp  vmlinuz
bin   etc   initrd.img.old  lost+found  opt   run   srv   usr  vmlinuz.old
boot  home  lib
/root  & /home Actually in Linux there are three things called root. Above root of the directory tree is a one. Next we have a super user called 'root'. Also there is a special directory as /root that's owned by root user. this one is equal to home folder of root. Normal users have there own home folders in /home. If your user name is 'us3r'   your home folder will be /home/us3r.  I think you ma have see that in many times there is a flag called root.txt in CTF s inside /root. So if you want to get that flag you need to root the mashing. /var Hear you can find files those have a variable size. It's a common thing that htdocs folder of server is located in this directory. In early versions that was /var/www and now that's /var/www/html. Also log files are located inside this directory. (/var/logs) 
thilan@bt:~$ ls /var
backups  lib    lock  mail  run   spool  unicornscan
cache    local  log   opt   snap  tmp    www
/tmp This is the place for temporary files .Usually this folder has the write permission for all. You may find that when uploading a shell to server we first change current directory to /tmp. /sys /usr This directory comes with  commands and data files .There are many sub application files in /usr/share folder. 
thilan@bt:~$ ls /usr
bin  games  include  lib  lib64  local  sbin  share  src  var

thilan@bt:~$ ls /usr/share
accountsservice              gvfs                            pev
aclocal                      hal                             php7.2-common
aclocal-1.16                 hamster-sidejack                php7.2-json
adduser                      hashcat                         php7.2-mysql
afl                          hashcat-utils                   php7.2-opcache
aglfn                        help                            php7.2-pgsql
airgeddon                    hexorbase                       php7.2-readline
alsa                         hfsprogs                        php7.2-sqlite3
android-framework-res        highlight                       pipal
ant                          homebank                        pixmaps
/etc This folder is responsible for holding all configuration files. If you want to tweak your Is or programs you may need to go there.For a example to change sources in apt you want to edit /etc/sourses.d 
thilan@bt:~$ ls /etc
adduser.conf            gimp                  menu-methods       rmt
adjtime                 glvnd                 mercurial          rpc
aliases                 gnome-system-tools    mime.types         rsyslog.conf
alsa                    gnuradio              miredo             rsyslog.d
alternatives            groff                 miredo.conf        samba
amap                    group                 mke2fs.conf        sane.d
anonsurf                group-                modprobe.d         scalpel
apache2                 grub.d                modules            screenrc
apm                     gshadow               modules-load.d     securetty
apparmor                gshadow-              mono               security
apparmor.d              gss                   motd               selinux
apt                     gtk-2.0               mpv                sensors3.conf
arpwatch                gtk-3.0               mtab               sensors.d
/bin This directory contains all executable binary files required for booting the system. Commands those run in single user-mode such as ls, ping, chmod are also located hear. 
thilan@bt:~$ ls /bin
bash                findmnt         netstat        su
btrfs               fsck.btrfs      networkctl     sync
btrfsck             fuser           nisdomainname  systemctl
btrfs-find-root     fusermount      ntfs-3g        systemd
btrfs-image         getfacl         ntfs-3g.probe  systemd-ask-password
btrfs-map-logical   grep            ntfscat        systemd-escape
btrfs-select-super  gunzip          ntfscluster    systemd-hwdb
/lib Hear you can find system libraries and kernel modules. /opt This folder can be used to hold custom program files. If you wanted to install sublime manually you may copy files to /opt/sublime. 
thilan@bt:~$ ls /opt
car-hacking  firmware-mod-kit  sublime_text  Teeth  xplico
/dev This directory Contains device related files for all hardware devices . If you are going to mount a new drive you may need this directory. 
thilan@bt:~$ ls /dev
agpgart          kmsg                serial    tty20  tty47    uhid
autofs           log                 sg0       tty21  tty48    uinput
block            loop-control        sg1       tty22  tty49    urandom
bsg              lp0                 sg2       tty23  tty5     vcs
btrfs-control    mapper              sg3       tty24  tty50    vcs1
bus              mem                 sg4       tty25  tty51    vcs2
cdc-wdm0         memory_bandwidth    shm       tty26  tty52    vcs3
cdrom            mqueue              snapshot  tty27  tty53    vcs4
cdrw             net                 snd       tty28  tty54    vcs5
So I hope you got an idea about Linux directory tree. Thank you for reading. Just leave a comment if somthin is unclear or wrong. :-)