These are usually aluminum oxide crystals and when a cream containing them is used they feel like fine grains of sand on your face. These particles are found in at-home microdermabrasion creams, but  dermatologists apply these same crystals in microdermabrasion machines for buffing or dermabrading the outer layer of skin, to smooth out and lessen the appearance of shallow acne scars, give tired skin a glow, or to minimize the appearance of fine lines.
 
Caution: If you are too aggressive with the microdermabrasion creams, you will turn red (or, in extreme cases, briuse), but you will see any potential irritation immediately and should stop as soon if there is some redness. If this should happen, do not apply any AHA’s (glycolics) or retinoids to your skin until any redness subsides.