DNA is a double helix made of nucleotides.

The sugar-phosphate backbones are on the outside.

The 2 strands are oriented in antiparallel directions: the 3' carbon of one nucleotide forms a covalent bond to the phosphate group (on the 5' carbon) of the next nucleotide.

Complementary pairs of nitrogenous bases are held together on the inside by hydrogen bonds that follow these base pairing rules:

Cytosine (C) pairs with Guanine (G) by forming 3 hydrogen bonds.
Adenine (A) pairs with Thymine (T) by forming 2 hydrogen bonds.

Note: RNA can also base-pair with DNA using the same base pairing rules, except A pairs with U.

Review:
Base-pairing Exercise: