Every cell in the body with a nucleus (a compartment in most cells) has the same complete set of genes. A gene is made of DNA (deoxyribonucleic acid) and is basically a type of genetic instruction. Those instructions can be used for making molecules and controlling the chemical reaction of life. Genes can also be passed from parent to offspring; this is inheritance.
Some genes are active ('on') in some tissues and organs but not in others. This is what makes the difference between a liver cell and a lung cell. Genes are turned on and off during development and in response to environmental changes, such as metabolism and infection.
A few definitions:
DNA (deoxyribonucleic acid) - a double-stranded molecule made up of four building blocks called nucleotide bases (different chemicals that are abbreviated A, T, C, and G) that are arranged in a certain order throughout a genome. The human genome has 3 billion pairs of bases. The order of these nucleotides are critical to the accuracy of the instructions of a gene. The most important molecules encoded by genes are RNA and proteins.
Gene - the functional and physical unit of heredity passed from parent to offspring. Genes reside on chromosomes. If a gene is like a chapter in a book, the chromosome is the book itself. Proteins and RNA influence how an organism looks, how well its body metabolizes food and fights infection, and even how it behaves. The number of human genes is about 20,000-25,000. Different genes can vary in length and cover thousands of bases.
Genome - All the DNA contained in an organism or a cell.