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Although each organism's DNA is unique, all DNA is composed of the same nitrogen-based molecules. So how does DNA differ from organism to organism? It is simply the order in which these smaller molecules are arranged that differs among individuals.

Source: https://www.nature.com/scitable/topicpage/dna-is-a-structure-that-encodes-biological-6493050/

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DNA Is a Structure That Encodes Biological Information

A schematic shows three double-stranded DNA molecules against a white background. The sugar-phosphate backbone of the molecule in the middle of the frame is represented as a segmented grey cylinder coiled into a double helical shape. Base pairs are represented as twisted rectangular prisms connecting the two strands and resemble rungs on a ladder, each half of the rungs being a different color, either blue-orange or green-red. The different colors represent the different nucleotide bases that make up each pair. DNA molecules on the lower left and upper right resemble the molecule shown in the center, but are shown only in grey scale.
What do a human, a rose, and a bacterium have in common? Each of these things — along with every other organism on Earth — contains the molecular instructions for life, called deoxyribonucleic acid or DNA. Encoded within this DNA are the directions for traits as diverse as the color of a person's eyes, the scent of a rose, and the way in which bacteria infect a lung cell.
DNA is found in nearly all living cells. However, its exact location within a cell depends on whether that cell possesses a special membrane-bound organelle called a nucleus. Organisms composed of cells that contain nuclei are classified as eukaryotes, whereas organisms composed of cells that lack nuclei are classified as prokaryotes. In eukaryotes, DNA is housed within the nucleus, but in prokaryotes, DNA is located directly within the cellular cytoplasm, as there is no nucleus available.

But what, exactly, is DNA?

In short, DNA is a complex molecule that consists of many components, a portion of which are passed from parent organisms to their offspring during the process of reproduction. Although each organism's DNA is unique, all DNA is composed of the same nitrogen-based molecules. So how does DNA differ from organism to organism? It is simply the order in which these smaller molecules are arranged that differs among individuals. In turn, this pattern of arrangement ultimately determines each organism's unique characteristics, thanks to another set of molecules that "read" the pattern and stimulate the chemical and physical processes it calls for.