This is the most common form of Leukonychia, small white spots on the nails. Picking and biting of the nails are a prominate cause in young children and nail biters. In most cases, when white spots appear on a single or a couple of fingers or toes, the most common cause is injury to the base (matrix) of the nail. When this is the case, white spots disappear after around eight months, which is time necessary for nails to regrow completely.
http://www.salongeek.com/health-safety-n… (page with picture).
White spots showing up on all or nearly all nails for longer periods of time (months and years) can be due to one of several reasons. One common explanation is zinc deficiency.