Double helix: The DNA looks like a long twisted ladder. The sides of the ladders are composed of phosphates and sugars.
Base pairs: The rungs of the ladder are composed of four bases in pairs that specify genetic instructions – adesine (A), thymine (T), guanine (G) and cytosine (C). "A" always pairs with "T", and "G" always pairs with "C".
Locus: The location of a particular gene or DNA sequence on a chromosome.
Allele: the alternative form of a gene or DNA sequence that occurs at a given locus. Some loci have only one allele, some have two, and some have many alternative forms. Alleles occur in pairs, one on each chromosome.
For a more in-depth explanation of genetics and patterns of human variation, read Human Genetic Variation: The Mechanisms and Results of Microevolution by geneticist Jeffrey C. Long.