Etiqueta vs. pegatina vs. calcomanía

While the Spanish words 'etiqueta', 'pegatina', and 'calcomanía' all carry the basic meaning of 'sticker', they each have unique nuances and contexts in which they are used.

Etiqueta

The word 'etiqueta' often relates to label or tag like price or information tags on goods, or a name tag on someone’s clothing. It implies an official or functional reference.
La etiqueta en la camisa dice que está hecha de algodón.
(The label on the shirt says that it is made of cotton.)
Por favor verifique la etiqueta de precio antes de comprar.
(Please check the price tag before buying.)

Pegatina

'Pegatina' relates more to sticker in general context and can be used for decorative, promotional, or informational purposes that is stuck onto something.
Vamos a usar pegatinas para decorar la tarjeta.
(We're going to use stickers to decorate the card.)
Las pegatinas electorales se están volviendo muy populares.
(Election stickers are becoming very popular.)

Calcomanía

'Calcomanía' often refers specifically to a type of sticker or decal that is transferred onto another surface by rubbing or pressing. It is mostly associated with temporary tattoos or vinyl car decals.
Mi hijo quiere una calcomanía de dinosaurio en su brazo.
(My son wants a dinosaur decal on his arm.)
La calcomanía del coche dice que es un híbrido.
(The car decal says it's a hybrid.)

Summary

'Etiqueta' is usually used in formal contexts and refers to functional labels, 'pegatina' has a broad use and refers generally to any type of sticker, while 'calcomanía' relates to transitory decals often placed on skin or vehicles.