The opposite-direction arrangement of the two DNA strands is called antiparallel. One strand runs from its 5′ end to its 3′ end, while the complementary strand runs from 3′ to 5′.
These directions come from the numbering of carbon atoms in the deoxyribose sugar. The phosphate backbone links the 5′ carbon of one nucleotide to the 3′ carbon of the next, giving each strand a chemical direction. The strands lie alongside one another, but their backbones point in opposite orientations.
Antiparallel alignment allows complementary bases to pair in the familiar DNA double helix: adenine pairs with thymine, and guanine pairs with cytosine. It also explains an important feature of replication. DNA polymerase can add nucleotides only to a 3′ end, so one new strand is made continuously while the other is assembled in shorter Okazaki fragments.
“Parallel” is a common mix-up because the strands run side by side, but parallel strands would point in the same chemical direction. “Antiparallel” refers specifically to their 5′-to-3′ orientation, not to the helix’s right- or left-handed twist.