Alleles account for variations in characters.  An allele is one of several versions of a gene - variations of a given trait if you will either dominant or recessive.

For each character, organisms inherit 2 alleles, one from each parent.  A character is a heritable feature.

If the 2 alleles differ, then the dominant allele is fully expressed and the recessive allele has no noticeable effect on appearance.

The 2 alleles segregate during gamete production (meiosis).