Cáscara vs. piel
The Spanish terms 'cáscara' and 'piel' both refer to the outer covering of something, commonly fruits. While they can sometimes be used interchangeably, there are subtle distinctions in their usage and connotations.
Cáscara
The word 'cáscara' generally refers to the harder, more brittle outer skin of certain fruits, nuts and eggs. It usually implies that this shell is not normally eaten.
La cáscara de naranja contiene aceites esenciales.
(The orange peel contains essential oils.)
Después de comer el huevo, tiré la cáscara al compost.
(After eating the egg, I threw the shell into the compost.)
Los cacahuetes todavía tienen su cáscara.
(The peanuts still have their shells.)
Piel
'Piel' refers to the outer layer or skin of a fruit, animal, or human. When used for fruits and food items, it often implies that this part can be thin and sometimes eaten, although not always.
La piel de la manzana es rica en nutrientes.
(The apple skin is rich in nutrients.)
Mucha gente prefiere pelar las patatas, pero yo cocino con la piel para más fibra.
(Many people prefer to peel potatoes, but I cook with the skin on for more fiber.)
Es importante cuidar la piel para mantenerla saludable.
(It's important to take care of the skin to keep it healthy.)
Summary
In summary, 'cáscara' is usually used for harder protective coverings that are commonly not eaten, such as those found on eggs, nuts, or thick-skinned fruits like oranges and melons. In contrast, 'piel' generally refers to softer skins that can potentially be consumed and are found on animals and thinner-skinned fruits such as apples and grapes.