Different forms of a gene are called alleles. Individual members of a population have different alleles. Together, all the alleles for all the genes in a population constitute the "gene pool" of the population. Through reproduction, individuals pass their genes on to the next generation. If considering only the effect of genetic drift, the larger the population is, the more stable the frequency of different alleles in the gene pool will be over time. In small populations allele frequencies are likely to change rapidly and dramatically over very few generations, or "drift," because of chance events. This rapid change can occur in small populations because each individual's alleles represent a large fraction of the gene pool, and if an individual did not reproduce it could have a much larger effect than in the case of an individual in a large population not reproducing. Also, alleles that are found infrequently are more likely to be lost due to random chance.
After many generations, if only genetic drift is operating, populations (even large populations) will eventually contain only one allele of a particular gene, becoming "monomorphic," or fixed for this allele.