The following article was sourced from a Wikipedia page at the following address: http://en.wikipedia.org/wiki/Primer_(cosmetics)
A cosmetic primer is a cream or lotion applied before another cosmetic to improve coverage and lengthen the amount of time the cosmetic lasts on the face.
There are different kinds of cosmetic primers such as foundation primer, eyelid primer, lip primer, and mascara primer.
A foundation primer may work like a moisturizer, or it may absorb oil with salicylic acid or aid in creating a less oily, more matte appearance. It aids in applying the foundation more evenly and smoothly, and increases the longevity of the foundation. Some contain antioxidants such as A, C, and E, or other ingredients such as grape seed extract and green tea extract. There are water-based and silicon-based foundation primers. Ingredients may include cyclomethicone and dimethicone. Some primers do not contain preservative, oil or fragrance. Some may also have sun protection factor (SPF). Some foundation primers are tinted to even out or improve skin tone or colour. Others give a pearlized finish to make the complexion more light reflective. There are also foundation primers which are mineral-based primers, which contain mica and silica.
Eyelid or eye shadow primers are similar, but made specifically for use near the eyes. An eyelid primer may help even the colour of the lid and upper eye area, may reduce oiliness, may add shimmer, or inversely may mattify. Eye primers aid in the smooth application of eye shadow, prevent it from accumulating in eyelid creases, and improve its longevity. There are many different brands of eye shadow primers on the market. Two popular eye shadow primers include Urban Decay Primer Potion and Too Faced Shadow Insurance. Eye shadow primers are applied to the eyelid and lower eye area prior to the application of eye shadow. They even out the skin tone of the eyelids hide eyelid veins, and smooth out the skin of the eyelids. Eye shadow primers help with the application of eye shadows. They intensify the colour of the eye shadows and keep them from smearing or creasing by reducing the oiliness of the lids. The effect of eye shadow primers is not limited to eye shadows. They also work for eye liners and eye shadow bases.
Mascara primer is sometimes colourless. It usually thickens and/or lengthens the lashes before the application of mascara for a fuller finished look. It may also help keep mascara from smudging or flaking, and some claim to improve the health of the lashes.
Lip primers are intended to smooth the lips and help improve the application of lipstick or lip gloss, although exfoliating the lips is often recommended before applying. They also are intended to increase the longevity of lip colour, and to prevent lipstick from "feathering", that is, smearing past the lip vermilion, and especially from migrating into any fine lines around the lips.
To read more about primers, please click on the following link: http://en.wikipedia.org/wiki/Primer_(cosmetics)