Nude can be:
Topless - page 3 style
Nude - naked with legs closed
UK Mens mag - posing with legs open
US Mag - posing holding your labia open
Continental - insertion of toys or own fingers
Usually topless, often playboy poses (open leg) done in bright conditions. Taken in bright settings, sometimes rooms or outdoors, not provocative but more classy, standard poses to show the body in a pin up kind of way.
Posing nude normally in dark conditions, sometimes as silhouettes. These are artistic interpretations of the models body, which can be very innocent, it is a way of the photographer getting the shape and his message across in the picture, these are not to be provocative, just to be expressive.
Modeling that is sexually artistic, not pornographic. Provocative, titilating, maybe a picture that tells a story with sexual meaning and content. Props may well be used to help portray a storyline to the pictures.