Demi- and semi-permanent colors generally differ from permanent color in that they:

Demi-permanent and semi-permanent colors are generally less long-lasting than permanent color: they deposit color and fade gradually over a number of shampoos, and they typically do little or no lifting (lightening) of the natural pigment, since they use little or no developer (or a low-strength one). Permanent color, by contrast, can lift and lasts until it grows out. Knowing the broad differences among temporary, semi-, demi-, and permanent color (longevity and lift) is standard color theory, with specifics learned through training.