Sello vs. estampilla
While both sello and estampilla can be translated as "stamp" in English, they are not always interchangeable. Sello is a much broader and more common term, while estampilla is more specific, with its usage often depending on the region.
Sello
A2This is a very broad term. It can refer to a postage stamp, but also to any official mark, a rubber stamp, a company seal, a wax seal, or even a figurative or distinctive mark, like a personal style or a record label.
El notario puso su sello en el documento oficial.
(The notary put his seal on the official document.)
Necesito comprar un sello para enviar esta postal a mi familia.
(I need to buy a stamp to send this postcard to my family.)
La factura no es válida si no tiene el sello de la empresa.
(The invoice is not valid if it doesn't have the company's stamp.)
Ese director de cine tiene un sello muy personal en todas sus películas.
(That film director has a very personal stamp in all his movies.)
La carta antigua estaba cerrada con un sello de cera roja.
(The old letter was closed with a red wax seal.)
Estampilla
B1This word almost exclusively refers to a postage stamp, the small adhesive paper used to pay for mail services. It is the term most associated with stamp collecting (filatelia). While used in Spain, it is much more common in many parts of Latin America. Occasionally, it can also refer to a tax or revenue stamp (estampilla fiscal).
Pega la estampilla en la esquina superior derecha del sobre.
(Stick the stamp on the top right corner of the envelope.)
Mi abuelo tiene una colección de estampillas de todo el mundo.
(My grandfather has a collection of stamps from all over the world.)
Fui a la oficina de correos a comprar una estampilla para una carta internacional.
(I went to the post office to buy a stamp for an international letter.)
Para legalizar el documento, tuve que adherir una estampilla fiscal.
(To legalize the document, I had to affix a tax stamp.)
Esa estampilla conmemorativa es muy rara y valiosa.
(That commemorative stamp is very rare and valuable.)
Summary
In essence, sello is the general, all-purpose word for any type of stamp, seal, or identifying mark. Estampilla is specific and almost always means a postage stamp for mail. If you are in Spain and referring to a postage stamp, sello is more common. In many Latin American countries, estampilla is the preferred word for a postage stamp. Think of it this way: every estampilla is a type of sello, but not every sello is an estampilla.