du - Commonly used for Disk Utility of individual files / folders
df - Overview of Disk file system allocations

Get Folder sizes on system root level

du -sh /*

# -s to give only the total for each command line argument.
# -h for human-readable format
# /* expands to all directories (and files) in specified path (which is system root in the example)

Sort folders by sizes in descending order

du -sh /* | sort -hr

Check File System Usage

df -h