I have an OFFICAL withdrawal "W" on my record, vs. an UNOFFICAL withdrawal "UW". Unofficial is basically due to the fact that you just stopped going, and doing tests etc.
For my official withdrawal I had appendicitis and knee surgery in the same semester, and surgery/recovery for both fell on test days. The professor wouldn't let me make it up, so I got a note from my doctors and received an official withdrawal. According to my councilor they cannot assume anything, either positive or negative with an official withdrawal. It could be something like being in the reserves and having to go to Iraq, or your parents died, or any number of very legitimate things, and the dental school (or whoever)legally cannot question it.
However, an audit, or an unofficial withdrawal is considered an "F", and is according to AADSAS. Once again, according to my councilor, this has been argued in court, and the schools have won. I have both a W, and UW on my record.
On my AADSAS GPA the W doesn't contribute to, or hurt it(however it may not make an influence in my interview or whatever, they may question it), and the UW translates into a fail.