A quick way to get a summary of the available and used disk space on your Linux system is to type in the df command in a terminal window. The command df stands for "disk filesystem". With the -h option (df -h) it shows the disk space in "human readable" form, which in this case means, it gives you the units along with the numbers.
The du command on the other hand shows the disk space used by the files and directories in the current directory. Again the -h option (df -h) makes the output easier to comprehend.
Thanks Juergan, this is really easy to digest!