A single-entry Schengen visa allows you to enter a Schengen country only at one given time during a period of 180 days and for a maximum of 90 days. Once you exit the Schengen country, you are not allowed to return and have to apply for a new visa altogether. A multiple-entry Schengen visa allows you to enter multiple Schengen countries within the same span of 180 days and for a maximum of 90 days. It saves you the effort of doing paperwork again and again.