Decal, pressure sensitive label, self adhesive label, labelstock are many name of same product. Decal is a label that is coated with an adhesive which is self sticking (or pressure sensitive).
Unlike a label that won't stick by itself. i.e. a label to which moisture (gummed paper), heat (heatseal) or glue(plain paper) has to be applied. A Self Adhesive Label is precoated with an adhesive which is protected by a backing material or liner.
When this backing paper is removed, the label will stick to a surface against which the adhesive side is placed.
The adhesive does not need to be activated by moisture or heat. Only a small amount of pressure is needed to cause adhesion because the label is pressure sensitive!