Phosphate backbone

A phosphate backbone usually refers to a double stranded DNA molecule, where the sequence of a phosphate group and deoxyribose are connected in sequence by phosphodiester bonds. The phosphate group always sticks out of the double helix of DNA due to the arrangement of the bases within the helix, and because phosphate groups are found throughout the entire DNA molecule, sticking outside the helix, it is referred to as a 'phosphate backbone' (imagine the bones that protude through the skin along your spine).