Duplex DNA is a long polymeric species made up of two individual molecules. Each strand of DNA is made of repeating units, called nucleotides. Complementary strands of DNA have nucleotides which interact favorably via base pairing. The shape and hydrogen bonding between bases ensures complementarity and their planarity allow them to "stack" on each other. But other forces are important in maintaining the structure.
The stick representation above is the actual structure of a very short stretch of DNA