It consists at most of a long dress with long sleeves and a white capelet, and typically stars the distinctive nun coif and veil (and may also include a wimple, tabard, underdress, and so on). The habit is typically white-on-black, with black the predominant color, but they can also be wholly white, or white-on-blue. Color can depend on the religious order which the nun is a part of. They can also be wholly black, which is the common choice for Orthodox nuns.
The traditional habit can be accentuated with minor additions, but not so much that it detracts from the basics (ex. Sister Claire and Double as opposed to Specter and Index), and some degree of modesty must be adhered to (ex. not too tight-fitting, no cutouts, no openings where they wouldn't normally be, no side slits which reveal skin or undergarment, no overtly sexual depictions of certain undergarments, etc).