DU, DF , Quota

du (abbreviated from disk usage) is a standard Unix program used to estimate the file space usage; space used under a particular directory or files on a file system.

Sum of directories in Kbytes:

$ du -sk *
2800344 directoryOne
4270554 directoryTwo

du -sh Documents/
409M    Documents

disk usage of all subdirectories and files including hidden files within the current directory (sorted by filesize) :
$ du -sk .[A-z]* *|sort -n

df (abbreviated from disk free) is a standard Unix computer program used to display the amount of available disk space for filesystems on which the invoking user has appropriate read access, df is usually implemented by reading the mtab file or using statfs.
$ df -k
Filesystem    1024-blocks      Free %Used    Iused %Iused Mounted on
/dev/hd4            32768     16016   52%     2271    14% /
/dev/hd2          4587520   1889420   59%    37791     4% /usr
/dev/hd9var         65536     12032   82%      518     4% /var
/dev/hd3           819200    637832   23%     1829     1% /tmp
/dev/hd1           524288    395848   25%      421     1% /home
/proc                   -         -    -         -     -  /proc


/dev/hd10opt        65536     26004   61%      654     4% /opt