There are two types of formatting. Quick format and Full format.
quick format is quick. To make the formatting process quick, the drive is not checked for bad sectors. Anyone looking at the hard drive or storage device would not see any data and assume the drive is erased. Unfortunately, the files are actually still there and the volume could be re-built, to gain access to the files again.
full format runs an additional step that checks the hard drive for any bad sectors. This check is what makes the full format take so much longer than a quick format. Unfortunately, like the quick format the files still exist and the volume could be re-built to gain access to any of the files on the drive again.