Different distributions have different directory structures, despite attempts at standardization such as the the Linux Filesystem Hierarchy Standard (FHS) organization.
Important directories
/bin -
essential UNIX commands such as ls, etc. Should contain all binaries needed to boot the system or run it in single-user mode
/boot -
files used during booting and possibly the kernel itself are stored here
/dev -
contains device files for various devices on system
/etc...
