proc: Process information
Directory /proc is very special in that it is also a virtual filesystem. It is sometimes referred to as the process information pseudo-file system. It doesn&#39;t contain &#39;real&#39; files, but rather, runtime system information (e.g. system memory, devices mounted, hardware configuration, etc). For this reason it can be regarded as a control and information center for the kernel. In fact, quite a lot of system utilities are simply calls to files in this directory. For example, &#39;lsmod&#39; is the same as &#39;cat /proc/modules&#39; while &#39;lspci&#39; is a synonym for &#39;cat /proc/pci&#39;. By altering files located in this directory, kernel parameters may be read/changed (sysctl) while the system is running.
The most distinctive facet about files in this directory is the fact that all of them have a file size of 0, with the exception of kcore, mounts and self.
/root: Administrator directory
Home directory of the System Administrator, &#39;root&#39;. This may be somewhat confusing, (&#39;/root under root&#39;) but historically, &#39;/&#39; was root&#39;s home directory (hence the name of the Administrator account). To keep things tidier, &#39;root&#39; eventually got his own home directory. Why not in &#39;/home&#39;? Because &#39;/home&#39; is often located on a different partition or even on another system and would thus be inaccessible to &#39;root&#39; when - for some reason - only &#39;/&#39; is mounted.
/sbin: System binaries
UNIX discriminates between &#39;normal&#39; executables and those used for system maintenance and/or administrative tasks. The latter reside either here or - the less important ones - in /usr/sbin. Programs executed after /usr is known to be mounted (when there are no problems) are generally placed into /usr/sbin. This directory contains binaries that are essential to the working of the system. These include system administration as well as maintenance and hardware configuration programs. GRUB (the command), fdisk, init, route, ifconfig, etc., all reside here.
/srv: Service data
Site-specific data which is served by the system. The main purpose of specifying this is so that users may find the location of the data files for a particular service, and so that services which require a single tree for read-only data, writable data and scripts (such as CGI scripts) can be reasonably placed. Data of interest to a specific user shall be placed in that user&#39;s home directory.
/tmp: Temporary files
This directory contains files that are required temporarily. Many programs use this to create lock files and for temporary storage of data. Do not remove files from this directory unless you know exactly what you are doing! Many of these files are important for currently running programs and deleting them may result in a system crash. On most systems, this directory is cleared out at boot or at shutdown by the local system. The basis for this was historical precedent and common practice.
/usr: Shareable, read-only data
While root is the primary filesystem, /usr is the secondary hierarchy, for user data, containing the majority of (multi-)user utilities and applications. /usr is shareable, read-only data. This means that /usr shall be shareable between various hosts and must not be written to, except in the case of system administrator intervention (installation, update, upgrade). Any information that is host-specific or varies with time is stored elsewhere.
Aside from /home/, /usr/ usually contains by far the largest share of data on a system. Hence, this is one of the most important directories in the system as it contains all the user binaries, their documentation, libraries, header files, etc. X and its supporting libraries can be found here. User programs like telnet, ftp, etc., are al