Multiple Alleles with Neutral Fitness
When there are multiple alleles for a gene, different individuals in a population will have different ones. An example is human blood types. In the ABO blood group system (where a person can have blood type A, B, AB, or O), three alleles of the same gene are responsible for all the possible types - one that codes for A, one that codes for B, and a nonfunctional one that codes for neither (usually designated with an O). Humans have two sets of chromosomes per person, thus two alleles per locus; if the two are AA or AO, the person has type A blood, if they're BB or BO, then it's type B, if it's AB, the type is AB, and if it's OO, the type is O.
In the case of ABO blood types, none of the alleles are particularly advantageous or disadvantageous over the others - all individuals in the population are equally able to survive and reproduce successfully (have equal evolutionary fitness) regardless of which type they have.
In a small population where all alleles of a gene have equal fitness, how they are proportioned will drift randomly from one generation to the next. In the absence of new mutations and new alleles migrating into the population, the general trend is for variability to go down, and eventually one of the alleles will be the only one that the population has (or, in the terminology of population genetics, one allele will become "fixed"). In a large population, the proportion of each allele from one generation to the next will be in Hardy-Weinberg equilibrium.