DNA is a large polymer consisting of two DNA strands that form a double helix.  Each strand is made up of bases.  In DNA, there are four different bases:

Adenine, Thymine, Cytosine & Guanine

Pairs of bases form chemical cross-links between the two strands of DNA to form the final double helix.  The base pairs always form a particular way:

- Adenine always pairs with Thymine (A --- T)

- Cytosine always pairs with Guanine (C --- G)

This process is called complementary base pairing.