In heterozygous individuals, the allele that is "expressed" (seen in individual’s appearance) is the "dominant" allele

The traits that are expressed make up your "phenotype"

The allele that is not expressed is the "recessive" allele

There are more than 2 alleles for some traits
Most traits, in fact, are affected by more than 1 gene

Sex-Linked Traits

In the case of genes located on the X chromosome, females would have the usual 2 alleles for each gene but males only have 1 X chromosome and thus 1 allele (the Y chromosome contains only a small number of genes related to gender)

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Shuffling the Deck
Chromosomes of offspring don’t have a half that’s clearly from dad and a half that's from mom
During duplication there is "crossing over" of bits of each half

Forelock: White forelock (dominant trait) vs. No white forelock (recessive trait)
A white forelock is a patch of white hair, usually located at the hairline.
Widow's Peak (below) is dominant over no widow's peak hairline.

Dimples: Dimples (dominant trait) vs. No dimples (recessive trait)
Dimples are natural dents in the face to the right or left of the mouth.  If a person has only one dimple, they should be counted as having dimples.Cleft chin is dominant over no cleft.

Thumbs: Straight thumb (dominant trait) vs. Curved thumb (recessive trait) When viewed from the side as in the illustration below, curved thumbs can be seen as part of a circle.

Mid-digit hair: Mid-digit hair (dominant trait) vs. No mid-digit hair (recessive trait)