Here is an excellent video from the Nature publishing group on their method of the year for 2009, induced pluripotent (iPS) cells. Follows are some select screenshots from the video (available on YouTube) with annotation:
How can we reproduce the properties of embryonic stem cells cheaply and somewhat efficiently?
By delivering just four factors (via retroviral vector), we can convert somatic cells into pluripotent ones. While not every exposed cell converts, we can convert enough cells to form self-sustaining colonies.
While the goal is to make fully pluripotent cells for experimentation and perhaps even therapy, cells may also be hijacked (e.g. moved towards a pluripotent fate and then differentiated into another somatic cell type). For example, neurons might be made from fibroblasts in this manner. While not a common technique, one round of infection can produce many cells that are partially reprogrammed (that have some but not all the traits of a truly pluripotent cell).