Linux and Unix file systems are organised in a hierarchical, tree-like structure. The highest level of the file system is the / or root directory. In the Unix and Linux design philosophy, everything is considered a file - including hard disks, partitions and removable media. This means that all files and directories (including other disks and partitions) exist under the root directory.
For example, /home/jebediah/cheeses.odt shows the correct full path to the cheeses.odt file that exists in the jebediah directory which is under the home directory, which in turn, is under the root (/) directory.
Underneath the root (/) directory, there is a set of important system directories that are commonly used across most Linux distributions. The following is a listing of common directories that are directly under the root (/) directory:
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/bin - important binary applications
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/boot - files that are required to boot the computer
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/dev - the device files
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/etc - configuration files, startup scripts, etc...
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/home - local users' home directories
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/lib - system libraries
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/lost+found - provides a lost+found system for files that exist under the root (/) directory
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/media - mounted (loaded) removable media such as CDs, digital cameras, etc...
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/mnt - mounted filesystems
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/opt - provides a location for optional applications to be installed
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/proc - special dynamic directory that maintains information about the state of the system, including currently running processes
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/root - root user home directory, pronounced 'slash-root'
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/sbin - important system binaries
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/srv - provides a location for data used by servers
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/sys - contains information about the system
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/tmp - temporary files
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/usr - applications and files that are mostly available for all users to access
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/var - variable files such as logs and databases