Genetic drift refers to a chance act acting over genetic variations or alleles previously distributed evenly over a population, drastically altering the distribution. For example, if I had 100 rabbits with coats white, brown, gray and black and suddenly a disease broke that killed all my black and brown rabbits, the genetic variation allowing for brown and black coats would have disappeared. This is a chance act, as the black and brown rabbits were not intentionally affected. Also there was no selective pressure for rabbits to develop one coat color, that's why the genetic variation was distributed evenly. This is different from artificial selection, as the eradication of black and brown coats was due to a random event, we did not select the coat color like people do for artificial selection.
