/bin - Binaries
/sbin - Binaries for root to run.
/lib - shared code
/usr - user binaries and their supporting files
/etc - Editable Text Configuration
/home/<USER> - files and software of a user
/dev - Device files, Hardware and Virtual devices
/opt - optional/add-on software packages
/boot - Contains linux kernel itself
/var - system-level variable data
/proc - Virtual Filesystem (in RAM), System & Process Info
Linux Directories Explained in 100 Seconds - Video/FireShip