What is cell differentiation?

We all begin life as a totipotent single cell: the zygote. Totipotency is when a cell can differentiate into any cell line of the human body. The zygote divides repeatedly and differentiates to produce many different kinds of cells. This is done through the process of differentiation or specialization, where a cell divides into two cells with different fixed characteristics, or role. Every cell in our body is specialized for a certain role and can't revert to a non-specialized state (apart from a few exceptions). For instance: red blood cells or erythrocytes' main role is to carry oxygen and carbon dioxide around the body.