df is the POSIX command for determining the amount of free disk space on a mounted filesystem. It presents its results in a simple columnar format:
Filesystem 1k-blocks Used Available Use% Mounted on
/dev/hda1 3526172 1139560 2207492 35% /
/dev/hda2 55337152 840 52525344 1% /home
otherbox:/home 53937632 21984112 29213632 43% /automount/otherbox.home
With the -h option, available in GNU df, the figures become more comprehensible:
Filesystem Size Used Avail Use% Mounted on
/dev/hda1 3.4G 1.1G 2.1G 35% /
/dev/hda2 53G 840k 50G 1% /home
otherbox:/home 51G 21G 27G 43% /automount/otherbox.home
The only other useful option is to list filenames or directories, each of which has the available space information printed for the filesystem it is associated with.
Although df is a very simple command, it is quite convenient, especially when working with large files. Its output, especially when the -h option is not used, is easily manipulable by other programs, and it fits with the Unix two-letter naming convention for convenience of typing. Probably the most convenient feature is that it defaults to displaying all filesystems, so that it is very easy to get a summary of the state of your machine. Nevertheless, it is not nearly as useful as its counterpart, du, as discussed at that node. | 
