Cellular differentiation is how a less specialized cell becomes a more specialized cell type. It is part of developmental biology. Different tissues have different kinds of organelles inside the cells.
Differentiation occurs many times during the development of a multicellular organism. The organism changes from a single zygote to a complex system of tissues and cell types. Differentiation is also a common process in adults: adult stem cells divide to make fully-differentiated daughter cells during tissue repair and during normal cell turnover.
Differentiation dramatically changes a cell's size, shape, metabolic activity, and responsiveness to signals. These changes are largely due to changes in gene expression. With a few exceptions, cellular differentiation almost never involves a change in the DNA sequence itself. It does involve switching off many genes not needed in a particular tissue. Thus, cells in different tissues may have very different physical characteristics despite having the same genome.
Cell potency is a cell's ability to differentiate into other cell types. A cell that is able to differentiate into many cell types is known as pluripotent. Such cells are called stem cells in animals and meristematic cells in higher plants. A cell that is able to differentiate into all cell types is known as totipotent. In mammals, only the zygote and early embryonic cells are totipotent, while in plants many differentiated cells can become totipotent with simple laboratory techniques.