Linux keeps every file in one tree under the root directory, /, and most distributions lay it out by the Filesystem Hierarchy Standard (FHS). The main directories are:
/bin: essential user commands
/boot: boot loader files and the kernel
/dev: device files
/etc: system configuration files
/home: user home directories
/lib: essential shared libraries
/media: mount points for removable media
/mnt: temporary mount points
/opt: optional software packages
/proc: a virtual file system with process information
/root: the root user's home directory
/run: runtime data
/sbin: system administration commands
/tmp: temporary files
/usr: user programs and data
/var: variable data such as logs and databases
Rewriting in plainer words…
This answer doesn't lend itself to a diagram - it reads best . No credits were charged.
