Drawing nudes is a lot more different than any other kind of drawings.
It consists in drawing a nude man or woman in a way that transcribes their humanity and personality.
In order to draw a good nude, some drawing skills are necessary but it is not enough. If you rely only on your eyes, chances are your drawing will look messy, disproportionate, and plain wrong. You will often need a basic knowledge of anatomy, such as the arrangement of the bones in the skeleton. This way you can see the model from the inside as well as the outside, helping you to understand what you see and hence, draw it better.
On top of that basic knowledge it is often very helpful to know the main muscles in the body to understand what happens under the skin of the model as he or she takes different uncommon postures. Without this knowledge it is unlikely that you would be able to resolve an awkward perspective of, say, a foot. However, this skeleton frame is not to be followed to the letter: some models are disproportionate according to the standards of the day : very long limbs, huge heads or very short torso. The anatomical knowledge is no substitute for acute observation.
A posture, as I practiced in first year of art school in Paris, can last from 10 seconds to 30 minutes in a 2 hours session. Bearing in minds that there is a point in your drawing when, whatever you do, you loose the spontaneity of the first shot and start to degrade your creation. One of the thing you must keep in mind is to know where to stop. That is true for any kind of drawings. Depending on the course you are following and your intention as an art student you may need the same postures for days on end, even weeks or more, especially when doing a sculpture, although it becomes increasingly rare to encounter truly anatomically correct nude drawings and sculptures.
A typical session, as I experienced it, would take place in a large room where 20 to 30 students would use an easel to support their rather large piece of paper on which they draw, historically the "raisin" format is used in France but A2 or A1 are common. There is a great freedom on the kind of paper or medium used to draw the nude, the medium could be standard paper pencils, Biro, fat leads pencils, markers, paint, graphite etc... The students congregate around a center small stage where the model takes his/her postures. More adventurous postures are possible if they are only meant to last less than a minute, otherwise a compromise has to be found between the teacher wishes, the interest of the pose and the endurance and comfort of the model. For most poses it is expected that the model will move involuntarily and that student will have to make corrections to their drawings. The quality of light is also important since a strong natural source of light will give nice shadows and relief to the model that will help the students to define it more accurately.
The models can be of any age or fitness although bodybuilders are more likely to be willing to show their body than others. During my time as an art student I saw pregnant women, women just older than me, weird exhibitionists, cultured middle aged women and even an funny old eccentric who would always have his little dog on display next to him. It was an enjoyable experience, as we get to know models throughout the year, we often ended up joking with them during the breaks when both students and models relaxed. Such sessions are very exhausting for students and model alike because of the concentration it demands from the students and the strain of staying immobile for the model.
Models were semiprofessionnals or plain amateurs with good self confidence and got paid a poor salary by the school (as of 1996). A tradition in my school was that, because of the poor pay the model got, each student had to give him the equivalent of the price of a cup of coffee (5 FRF, around 0,50 GBP or 0.7 USD) for each session. I have been told that in the 1960s, at "Les Beaux arts" (a famous art school in Paris) models were taken at random amongst the students themselves but this practice is definitely on the decline.
Astonishingly enough I have never heard of a single incident during these session anywhere in the school. This should be surprising in an art school full of 18 years old. Of course, when a bodybuilder posed, the girls were chatting happily during the break about his athletic body but that was about it.
One nice side effect of nude drawing sessions is that it somehow reconciles you with the human body, that is: imperfect, hairy, saggy etc. To see nude (almost) ordinary people counterbalances the perfection depicted in the media that represents, at best, 0.01% of the world population.
Leonardo da Vinci said of drawing in general that it was a spiritual exercise. I think with nude drawing you can come really close.