It is based on total family income after tax, and it's adjusted for family size. If you have a family income of $100,000 and one person, or a family income of $100,000 and four people, there's an adjustment made to account for that family's size. The individual family's income is then ranked from lowest to highest, and the population is divided into five groups of equal size: the bottom 20%, the next 20%, and so on. Among the quintiles I've shown you there, the bottom corresponds to the lowest or bottom 20% of the distribution, the middle the third, and the top the highest quintile.
On October 20th, 2009. See this statement in context.