Single-entry Schengen visa allows the holder to enter the Schengen country once only. Once the holder leaves the country, he or she cannot return.
Multiple-entry Schengen visa allows the holder to enter the Schengen area as many times as desired and then come back to the zone. Multiple entries are also categorized into 1-year, 3-year, and 5-year multiple-entry Schengen visas.