http://support.microsoft.com/kb/315672
"When you delete files or folders, the data is not initially
removed from the hard disk. Instead, the space on the disk
that was occupied by the deleted data is "deallocated." After
it is deallocated, the space is available for use when new data
is written to the disk. Until the space is overwritten, it is
possible to recover the deleted data by using a low-level disk
editor or data-recovery software."
It means the data is "marked for deletion", but the actual
deleting doesn't happen until the sector is used by another file
on a write. And this is why people give advice, to "stop using the disk",
meaning "stop writing to it", if something is deleted or
disappears accidentally. If you don't write to a disk, the
deleted data may all still be intact.
Paul