df is acronymic for disk free.
When this command is used with no parameters, it will display the amount of disk space free on each mounted device in a 512-byte per block count, as well as a percentage.
You may specify either a device path or plain directory path to retrieve the associated mount-point information. Also, the parameter -h may be used to have the output displayed in 'human-readable' format.
[light@splat ~]$ /bin/df Filesystem 1K-blocks Used Avail Capacity Mounted on /dev/ad0s1a 128990 104110 14562 88% / /dev/ad0s1f 257998 146 237214 0% /tmp /dev/ad0s1g 18809884 15014076 2291018 87% /usr /dev/ad0s1e 257998 236788 572 100% /var procfs 4 4 0 100% /proc
[light@splat ~]$ df /tmp Filesystem 1K-blocks Used Avail Capacity Mounted on /dev/ad0s1f 257998 146 237214 0% /tmp
[light@splat ~]$ df /dev/ad0s1f Filesystem 1K-blocks Used Avail Capacity Mounted on /dev/ad0s1f 257998 146 237214 0% /tmp
[light@splat ~]$ df -h / Filesystem Size Used Avail Capacity Mounted on /dev/ad0s1a 132M 107M 15M 88% /
 Deleting locked files
Scenario: MySQL is giving you error 28's, and you find /var is completely full. You find that Apache's logs have grown over 200MB, so you delete them. The 200MB this should free would put you well within limits... but for some reason, df still reports that /var is completely full. Why?
The problem is that even though you deleted the logfile, Apache still held a lock on it and thus the space cannot be reclaimed yet by the filesystem. This is not an error on [df]'s part; the space really is still being taken up until the lock is released. To fix it, you merely need to restart apache, (perhaps using Apachectl). Apache then releases the log file, df shows the space returned as free, and MySQL goes back to working.