While a summary is a way of concisely relating important themes and elements from a larger work or works in a condensed form, a synthesis takes the information from a variety of works and combines them together to create something new.
"Synthesis is similar to putting a puzzle together—piecing together information to create a whole. The outcome of this synthesis might be numeric, such as in an overall rating perhaps best typified in a quantitative weight and sum strategy, or through the use of meta-analysis, or the synthesis might be textual, such as in an analytic conclusion."
Synthesis. (2005). In Mathison, S. (Ed.), Encyclopedia of evaluation. Thousand Oaks, CA: Sage Publications, Inc. doi: 10.4135/9781412950558
Basic steps for synthesizing information: