What is meant by MS

Depends on the context. The first thing that springs to mind is the illness Multiple Sclerosis. Microsoft possibly?

MS - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/MS)

Before the name of a lady Eg. Ms.Nancy instead of Mrs.Nancy it means we do not know the marital status of Nancy is unknown. If we address a lady whose marital status unknown we can add Ms. before the name