Hola aivars,
Según el Diccio. Simon & Schuster's:

decal=calcomanía
sticker=etiqueta o marbete engomado


sticker
noun
2 a : one that adheres or causes adhesion b : a slip of paper with adhesive back that can be fastened to a surface

de·cal
noun
Etymology: short for decalcomania
Date: 1937
: a picture, design, or label made to be transferred (as to glass) from specially prepared paper

Merriam-Webster's

Buena suerte y saludos del Oso ¶:^)

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Note added at 2003-04-14 22:38:09 (GMT)
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Otra característica que las podría diferenciar entre sí es que las \'decals\' casi siempre son de vinil, para pegarse en cristales, que podrían estar expuestos al agua, por ejemplo y las \'stickers\' casi siempre son de papel.

En este sitio donde venden los dos tipos \"decals\" y \"stickers\" dice así:

***The vinyl waterproof decals and stickers (not vinyl)*** that we offer are simply the best product of it\'s kind available. A great selection of state, country, confederate, and military items is in stock now and all items ship within one business day. Order online now or call 1-800-552-4382 to order by phone.

http://www.unflags.com/flagdecsticp1.html

I don't think the difference is in the glue; in Spanish, I have seen "calcomania" applied to both what in English I would call stickers, and also decals. A decal is more official, such as the calcomania that you get from the Department of Transport to show that your car has passed the inspection for another year. A decal might (including in your text), bear some official logo.

A "sticker" type of calcomania, on the other hand is what my kids buy by the dozen to collect in their sticker albums. These would never be called decals.

Well, that's my answer, but a web search on the words (stickers, decals) turns up a different answer.

Apparently these are regionalisms. See the first reference: "What's the difference between stickers and decals?
It all depends what part of the country you're from -- but don't forget that there are also LABELS."

Another answer to the question is different (2nd reference): Stickers are generally paper based that are not weather proof and can be hard to remove. Decals on the other hand, are vinyl based, weather proof, and can be very easily removed even years later."
(This is in reference specifically to products made to decorate Jeeps.)

But if you search further, you can also find "vinyl stickers", showing that the above answer isn't definitive.

Ref:

Routledge Technical Dictionary

Gráfica ::: Guía Comercial TemperleyWeb
... ***Calcomanías y Etiquetas autoadhesivas***. Facciuto Serigrafía, Alcorta 236.
Tel: 4244-9188. Sevini Hnos. Vélez Sarsfield 1050. TFax: 4244-7763. ...
temperleyweb.galeon.com/grafica.htm - 23k - En caché - Páginas similares
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Business Printing Industry
... Anagraf - Fabricación y venta de ***calcomanias y etiquetas autoadhesivas***. Presenta
listado de productos y lista de precios. Andrés Amaya - Servicios gráficos. ...
www.da.com.ar/argentina/english/business/grafica.htm
****************************************

Gráfica ::: Guía Comercial TemperleyWeb
... ***Calcomanías y Etiquetas autoadhesivas***. Facciuto Serigrafía, Alcorta 236. Tel:
4244-9188. Papelera Didavi, Av. E. Perón 1585. Tel: 4245-2827 / 15-5308-2487. ...
www.temperleyweb.com.ar/grafica.htm - 24k - En caché - Páginas similares
***************************************

Suerte,

Ignacio