One of the most important questions about printing is what is the difference between decals, stickers or labels?

Stickers, labels, decals can be treated as the word tomato or tomahto.

The word Stickers, labels, decals changes but overall the product is the same with just multiple interpretations.
"What is the difference between a sticker, a label, and a decal?".
The short answer- It all depends on how you're gonna use it and what material and adhesive it has.

Stickers, labels, and decals have a definite grey area when it comes to defining these 3 sticky substrates; causing confusion when trying to select the best product for your needs!

Stickers: In general stickers are more graphical in its design (Such as logos, illustrations, and pictures you want to sure )
and are typically applied on a smooth surface of any size.
Labels: Labels, on the other hand, are used for identification or informative purposes (such as name tags and product packaging)
and are typically intended to go on a surface of a specific size (i.e on a mason jar lid measuring 3 inches; on the corner of a box labelled as fragile).
Decals: Decals are more graphical in its design the same as stickers just larger formats and intended to be applied on walls, glass, and floors. 

At its core, all products above are essentially the same thing: material with an adhesive backing.
Demonstrating only the end result of the purpose/application will define whether it is a sticker, label or decal.

One last factor to keep in mind is your application some Stickers, labels, decals will be made using paper and some other vinyl amongst other materials then an adhesive to match.
Vinyl Stickers, labels, decals will have a more durable and resistant adhesive than paper would due to the vinyl Stickers, labels, decals being made for outdoor use.
Some adhesives also will be more pressure sensitive than others (which essentially means the longer it stays and the more pressure is applied the stronger the bond).
Paper stickers also are less expensive than vinyl would and normally paper comes in smaller sizes/larger volumes whereas vinyl can be used for both smaller sizes and larger sizes as well as large & small volumes.