The nub is a 'pokey' out bit between the legs which will turn into genitals.

Generally if the nub is parallel to the spine or less than 30 degrees from the spine the nub will become the clitoris in a girl. If it is at a greater angle than 30 degrees it should become the penis for a boy.

Normally between 12-14 weeks the nub is visible with a good ultrasound machine, after then the genitals have generally formed. With my twins I had a scan at 16 weeks and they both had very obvious boy bits on show.