How do dna molecules vary from one species to another?

Organisms all possess DNA as their genetic material. What differentiates them (and their DNA) is the sequence of base-pairs within the DNA. The base-pairs are actually specific sequences of nucleotides (i. e. adenine, thymine, guanine and cytosine, labelled A, T, G, and C respectively) which encode genes. In other words, the DNA in each organism is made of these bases, but their sequences differ from organism to organism.