What are small sections of a DNA molecule that determine genetic traits called?

Bases. There are 4 different bases (in DNA not RNA)

They are Thymine, Adenine, Guanine and Cytosine. Thymine joins to Adenine and Guanine joins to Cytosine (connect by hydrogen bonds).

Different base sequences = triplet codes and different triplet codes change tertiary structure of proteins and alleles change.