What is the difference between common name and scientific name?

Common names are what the most of us use. Scientific names are what scientists use to differentiate, one from another in the classification system. The difference is precision: the scientific name is an attempt to remove as much ambiguity as possible, in order to avoid confusion. This level of precision is not usually needed in most circumstances and common names are easier to remember and pronounce. The important thing to remember is that everyone wants others to understand what they're saying, so everyone tries to use terms that have have the best chance to accomplish that goal.