Source Transformation - is the process of replacing a voltage source Vs, in series with a resistor R by s current source Is in parallel with a resistor R1 or vice-versa.
For more information on how to convert voltage source with series the resistor to current source with parallel the resistor click this link: http://www.dummies.com/how-to/content/simplify-circuit-analysis-by-transforming-sources-.html