Differentiation: The process by which unspecialized cells (like stem cells) become specialized into one of the many cell types that make up the body.
Embryonic Stem Cells: These are stem cells derived from human embryos. They can differentiate into almost every type of body cell.
Adult Stem Cells (or Somatic Stem Cells): These exist throughout the body after embryonic development and have been found in tissues such as brain, bone marrow, blood vessels etc. They usually give rise only specific types of similar tissue.