Humans, animals, plants, protozoa, bacteria, and even some viruses carry DNA inside them.
This DNA carries the genetic material or the "blueprint" which guides the cell into producing specific proteins that it needs for its survival and function. This processes is termed the "Central Dogma of Molecular Biology".
It has two major steps:
The process by which the DNA is transcribed into mRNA to be delivered to the Ribosomes which synthesize proteins in cells.
The process by which the mRNA is translated into Amino Acids and then into proteins by Ribosomes.
These two steps are identical in all living cells. And on top of that, the set of rules for re-writing (translating) the DNA sequence into the language of Amino Acid is the same in all living cells (with a few exceptions for some uncommon Amino Acids).
For example, the DNA sequence that codes for the Amino Acid Lysine will result in Lysine whether that sequence is read by a human cell, a bacterial cell, or a cactus cell.
This proves to us that the DNA provides the same core functional mechanisms to all living cells and that these cells all evolved from the same source.
I hope that answers your question.
References & Further Reading: