Disc wipers like Killdisk and Dban, eliminate the partitions and write characters, like 1's and 0's, to the disc. DoD disc wiping writes to the disc with multiple passes, 3 times, 4 times, 9 times, etc... Mulitple overwriting of the previous data makes it much harder to recover.
A format leaves the original address tables/markers and the overwriting is only done if you decide to reinstall an operating system, etc. Such data is possible to retrieve if one needs it bad enough..
If you're only trying to reinstall an operating system, then a format will usually do fine.. But if you're trying to wipe all the data from the drive for security reasons, then you need to use a disc wipe program.
Hope this helps.
What's the difference between a drive wipe app. like Killdisk and doing a Windows reformat of your HDD in setup?