If you have installed Linux on your system, then there are different pre-created directories. This directories are created by Linux as per its conventions to put its system created files. You can also put your own files in these directories.
Different distributions of Linux has slight difference in directory structure, but some important directories (like boot, var, etc) are common.
This is the root or main directory.
Bin stands for binary. This directory contains useful Linux software or command. It contains commands which are used by administrator and other users. This directory does not have any other sub directories.
Boot directory contains files required for booting Linux. It also stores data related to kernel loading. It contains grub loader files also.
Dev stands for device. This directory contains files related to hardware devices in Linux. It contains files related to hard disc, CD ROM, audio, input devices, etc.
Etc stands for etcetera. This directory is brain of Linux. It contains configuration files related to system and user packages. For example, it contains configuration for X11 which is system package and your apache web server which is a user package. If you have PHP installed, then configuration file of PHP (php.ini) is stored here.
Home directory is the directory for users. This is similar to My Document Folder in Windows. If you create a user in Linux, a folder will be created in the home directory for that user. Only administrator and the user can access the directory.
This directory contains various libraries which are used by different commands on Linux.
This directory is mount point for removable media. It is used for removable media. When you insert USB stick or CD, then a folder in the media directory is automatically created for that.
This directory is also use for mount point. But, it is used for mounting all devices and file system. Mounting is process by which you can make other device file system available to the Linux.
Opt stands for optional. This directory is for all optional or additional software. All add-on that are not part of default Linux distribution should go in /opt directory.
This directory is home directory of root user or super administrator user. For general user, home directory is created under /home/username directory. But for root user home directory is /root.
Sbin stands for system binary. This directory is identical to /bin directory. It stores binary executable files that can only be used for system maintenance or administrative task. Generally, it contains binary executable of booting, backup, restoration, etc.
Usr stands for unix system resources. This directory is one of the most useful directories in Linux operating system. It contains data like binary, documentation, library, etc. which can be shared among users. Basically, it contains read only data. This is user system resource directory on the Linux file system.
Var stands for variable. This directory is also one of the most important directories in the Linux. It is opposite of /usr directory. It contains all variables files. Basically, it sores data which dynamically changes like logs. This directory stores backup, db server middle-ware files, if running apache then .html files, etc.
This directory contains temporary files. You can use this directory to store your files temporary. For example, suppose you want to download and install a package. You can download the package in /tmp directory and install it. This directory is accessible from all users.
Sometimes, some running packages of Linux create temporary files in /tmp directory and store data for short terms use. Do not delete any files from /tmp directory if you are not sure of why it is created. It may crash any user program.