The command df is the one which you want. df does more that just list the mounted file systems, it also tells you where they are mounted, the file type, size of the mounted partition and how much of the partition is used.
For example 'df -h' on my computer gives the following

Filesystem Size Used Avail Use% Mounted on
/dev/hda1 18G 5.6G 12G 34% /
tmpfs 248M 0 248M 0% /dev/shm

This says that hda1 has 5.6 GB free