Specialized cells are cells that have a specific role in the body and help them fulfill that role, often having a specific shape and structure and specific amounts of different organelles like mitochondria.
Differentiation is the process by which a cell changes to become specialized, often involving changing the shape and structure of the cell as well as the combination of organelles that it contains.
Stem cells are cells that have the ability to repeatedly divide by mitosis and differentiate into different types of specialized cells.