This topic was discussed briefly on a page called "Late Drops and Graduate school"
Do an edit/find to get it.
Everybody has a different opinion on withdraws, but... I think you'll be ok if you still had a strong roster of challenging classes remaining. For instance, if you had 2 upper level physics and math classes remaining after the withdraw and did well in them, then you should be in good shape. But if that was your only challenging class that semester, then it will look like that semester was a waste.
As far as "explaining," I'd like to know what people mean when they refer to explaining. Do applications give us an area specifically to explain mishaps? Or do we have to throw them into our personal statement or something?
I have a few issues I'd like to get off my chest without giving too negative an impression, and I'm just wondering if I have to figure out a way to casually slip them in the SOP or if I could just write it all in some specially printed "Section 3B" haha