How do cells differentiate from each other? what sends a cell the message to become one kind of cell rather than another type?

Cell differentiation is a process by which stem cells give rise to different type of cells with different functions and located in different locations. it is is true that all cells in a particular organism have same composition of genetic material. however they perform different function because of the process of cell differentiation. cell differentiation results from expression of some genes inside the cell the dictate what type of cells should be synthesized.after gene expression proteins produced send the message to stem cells to indicate the type of cells to be manufactured.  

Cell differentiation is how cells came about in different types from being same embryonic cells.Genes are the one who decide this differentiation they send the required message to cell to which kind of specialised cell they have to develop. Cell are differentiated in their shape ,size and functions etc.Biologists still don't understand the complete process