During the Fall and Winter terms, you have until the end of the third week of class to drop without a W. From the end of the third week to the last day of class (as determined by the RO), you can drop with a W. (The only exception here is if it's your first full semester--i.e., Fall or Winter--at Michigan. In that case you will receive a W on your unofficial transcript, but not on your official transcript, i.e. the one sent externally to prospective employers, graduate schools, etc. unless you drop all of your classes, which would be considered a full term withdrawal)
Drop deadlines for the Spring and Summer half-terms are shorter. You can drop a class without a W until the end of the second week of class.
Please note: courses that have already ended or have been graded cannot be dropped.