Difference between a Virtual issue and a special issue? Can someone explain me the significance of Virtual issues, the difference between Virtual and Special issues? Also, under what conditions does a journal propose for a Virtual issue, does it have any impact on the Impact factor of the journal?
Virtual issues, which are also referred to as "collections" or by other names, are a set of papers that were already published in various issues of a journal and are centered around a single topic. This is usually done to highlight a certain topic or simply to help readers find papers on a topic. This is done on the editorial/website level and that is why it is "virtual" - usually no special content is created for this.

Special issues are issues in which the journal decides in advance about a certain topic, and invites submissions of papers on this topic. Often there is also an invited guest editor for the issue, who is an expert in the field. The final result could actually look similar to a virtual issue (a collection of papers on a single topic), but in a special issue these would be new papers submitted specifically for this issue. Sometimes there will also be some overview piece written by the editor.

Special issues might indeed affect the impact factor of the journal. For example, the journal Bioinformatics sometimes has a special issue in which the papers published are papers that were accepted to ISMB, a major bioinformatics conference. These papers probably get cited on average more than other papers in the journal, so it would increase the impact factor.