MikeS
Your right , when you delete a file, the file allocation table is flagged as the file deleted and the sector/track marked as free to be written to the actual data is still there untill its over written by the next write to the disk if the next write is allocated to that particular space. Getting that data back is a little tricky in the good old days of dos there was a utility called undelete which basically reversed the flag but now those utilities can only be found in special disk manager programes.
as somebody mention MBR or master boot record, this is not touched when doing a format, but can be re written by using FDISK to create and then remove a partition. Found out the hard way when a password was lost to a boot manager program