This short video produced by the World Bank provides a concise introduction of impact evaluations.  It explains that impact evaluations take into account the context in which the program is implemented to determine the real impact of the program.

"By using impact evaluations, we know how effective our programs are and can make good, cost-effective decisions about future programs to reduce extreme poverty and boost shared prosperity."

You can watch the video at the following link https://www.youtube.com/watch?v=HEJlT8t5ezU.  The video's description includes a link to more World Bank resources on impact evaluations.