We have learned that organisms inherit charcteristics which are encoded on the chromosomes in their cells. These chromosomes are passed from parent to child, and are reshuffled in each generation through sexual reproduction.
Some of the genes that code for a specific characteristic come in "different flavors". For example, this diagram shows some genes that determine eye color. They are all found on the same place on the same chromosome. The organism will have two chromosomes, and the eye color will be determined by the genes on those two chromosomes. There is a variety of eye colors in this population.
Genes that are variants of one another are called alleles. They have the same function, they are found in the same location, but what they determine (eye color here) is not expressed identically in all individuals.
Alleles probably started out as one gene with one mode of expression. Over thousands of reproductve copyings, a mistake was made, and the gene was changed a little. Such a change is called a mutation, and the individual whose phenotype (physical appearance or functioning) is different is called a mutant.
Many mutations cause changes that are not really noticeable, but that may turn out to be useful someday. For instance, there appears to be a mutation that allows people to become infected with HIV but not get sick or die. This mutation has never been valued or noticed, but perhaps it will turn out to be really imnportant. It is rare in our population, but could become more common over the next few centuries.
Some mutations are lethal. These cause the organism which inherits them to die prematurely. Some cause death in infancy or childhood, so these are not passed on. Some cause death in mid-adulthood or later: these may persist in a population because they are passed on before the individual affected dies.
The genes that are the code for some specific characteristic