DNA
DNA is the instruction manual that nearly every living cell carries inside it. Think of it as a very long recipe book written in a four-letter alphabet: the order of those letters spells out the directions a cell uses to build and run a body. Each of your cells holds essentially the same book, copied and tucked away when the cell was made.
Chemically, DNA stands for deoxyribonucleic acid. It is a long polymer built from repeating units called nucleotides, each carrying one of four nitrogenous bases — adenine, thymine, guanine, or cytosine. Two such strands wind around each other into a double helix, held together by specific pairing between the bases on opposite strands.
The sequence of bases is what carries information. Stretches of that sequence form genes, which can be read out to make proteins and functional RNA molecules; other stretches help control when and where that reading happens. Because DNA can be faithfully copied, this information is passed from cell to cell and from parents to offspring.
DNA is remarkably stable for a biological molecule, which is part of why it can store information across a lifetime and across generations. It is not perfectly stable, though — it is occasionally damaged or copied imperfectly, and those changes are the raw material of mutation and, over long timescales, of evolution.
A single human cell holds about two meters of DNA wound up tightly enough to fit inside a nucleus a few thousandths of a millimeter across.
The packing problem is solved by wrapping DNA around proteins and folding it into chromosomes.
DNA itself is the same chemical in all known life; what differs between a bacterium, an oak, and a human is mostly the length and the order of the letters, not the molecule.