Cuello vs. nuca
In Spanish, the words 'cuello' and 'nuca' are both used to refer to different parts of the neck area. However, each term refers to distinct aspects.
Cuello
'Cuello' refers to the entire neck in general from its base connected to the shoulders and torso up to where it connects with the head.
Me duele el cuello.
(My neck hurts.)
Se puso una bufanda alrededor del cuello.
(He wrapped a scarf around his neck.)
El jersey tiene un cuello alto.
(The sweater has a high neck.)
Nuca
'Nuca' specifically refers to the back part of the neck, situated between the head and shoulders.
Se tocó la nuca y notó que tenía fiebre.
(She touched the back of her neck and noticed that she had fever.)
El tatuaje en su nuca es muy pequeño.
(The tattoo on his back of his neck is very small.)
La pelota golpeó la nuca de Jorge.
(The ball hit Jorge on the back of the neck.)
Summary
'Cuello' and 'nuca' are two Spanish words referring different parts of neck. 'Cuello' tends to be used more generally for the entire neck area whereas 'nuca' denotes specifically to the back part of it.