The way it worked at the universities to which I went would be that if you withdrew during drop/add or the first week or two of classes, it'd drop off your record completely. If you withdrew after that, you'd generally get a W. If you withdrew after the midpoint of the semester, I believe you'd get a WF, which basically factors in as failing. Withdrawals in generally weren't counted in calculating your GPA, though (unless, as just mentioned, it was a WF).
If you had an emergency, you could withdraw with an I (incomplete) pending approval by the school/professor, and then work at making up what you missed the next semester.