It doesn’t differ much between species. In fact, you would be hard pressed to tell the difference between a human and a mushroom at the chromosomal level. The genetic foundations of life on earth share a basic similar structure, with only a relative handful of coding genes that account for the enormous variety of characteristics that classify different organisms. At the core, we’re all pretty much the same.
The bulk of it is junk. Around 98% of our DNA is called non-coding, or “junk” DNA. Where did all the refuse come from? Mostly from defunct genes and non-fatal replication errors, but also from centuries of viral activity that has been altering and rearranging our genes. Viruses operate primarily by sawing into particular areas in a gene sequence and inserting their own piece of DNA, using the host’s “machinery” to manufacture new copies of the virus. Sometimes the host cell survives the invasion, but carries the new piece of DNA with it as it replicates in the future. The new sequence doesn’t function, but stays in the family photo album permanently.
You have your own unique mutations. The gene-replication machine that is responsible for copying gene sequences is the extraordinary DNA Polymerase. However, mighty as it is, it makes a lot of mistakes. Individual people experience hundreds of gene mutations, beginning with egg and sperm and continuing throughout their lifetime. Most mutations are benign, but occasionally one can create changes in a critical sequence. These changes can be harmful or beneficial and are the also the microscale foundation of evolution.
Your cat might be two different cats. Actually, you might be too. DNA from two individuals can wind up as one. Chimerism is the presence of cell types from two different organisms within one individual. This occurs when twins implanted in the uterus accidently fuse together early in the embryonic stage. The condition is common in cats and often manifests in the bipolar coloration that characterizes certain breeds with split faces and different colored eyes. Though rare, this can happen in humans as well, and has been known to falsely negate paternity testing.
You have an astronomical amount of DNA wound up in your cells. DNA is microscopic and bound up in neat little piles within each cell, but if you could unwind every strand in your body, it would stretch to Pluto and back!