Gene expression, a conceptual illustration. Gene expression is the when a section of DNA is processed to produce a protein or other molecule that results in a particular characteristic.
Every cell in an organism contains every single gene. They are not all active (expressed) in each cell though. Different genes are expressed in different cells and also in response to different conditions. Cellular fate, or which type of cell a cell decides to become, is influenced by gene expression. Cells with the same genetic material can specialise into different cells by changing the expression patterns of genes. Epigenetics is the study of mechanisms that influence gene expression patterns without changing the underlying DNA sequence.