DNA is a double helix – two DNA strands wound around each other. The bases of each strand are on the inside of the helix, and a base on one strand pairs with one on the other in a very specific way. DNA has only four different bases:
Wherever we find an A in one strand, we always find a T in the other; wherever we find a G in one strand, we always find a C in the other. The strands are complementary. If we know the base sequence of one, we automatically know the sequence of the other.
The process of replication takes one strand (after they come apart) and enzymes build new partners for them using the old strands as templates following the Watson-Crick base-pairing rules. This process is called semi-conservative replication since one strand of the parental double helix is conserved in each of the daughter double helices.