The Filesystem Hierarchy Standard (FHS) defines a standard layout to organize various kinds of application and OS related data in a predictable and common way [1].

A basic knowledge of the FHS will help you to find application or OS related data more easily. If you are a developer, it also provides a good orientation for organizing your own applications in a maintainable way, e.g. as ubuntu package.

/bin – essential user commands

/boot – OS boot loader

/dev – devices (everything is a file principle)

/etc – system configuration

/home – user data

/lib – essentail shared libraries

/media – mount point for removable media

/mnt – mount point for temporarily mounted filesystems

/opt – add-on applications

/root – home of root

/run – run time variable data

/sbin – system binaries

/srv – data for services provided by the system

/tmp – temporary data

/proc – is a virtual filesystem

/usr – secondary hierarchy

bin – Most user commands
lib – Libraries
local – Local hierarchy (empty after main installation)
sbin – Non-vital system binaries
share – Architecture-independent data

/var – variable data

cache  – Application cache data
lib  – Variable state information
local  – Variable data for /usr/local
lock –  Lock files
log – Log files and directories
opt – Variable data for /opt
run – Data relevant to running processes
spool – Application spool data
tmp  -Temporary files preserved between system reboots

Find more

What about – /init.d ?

What does the .d stand for in directory names?

FHS in Debian