what is the molecular difference between Starch and carbohydrates?

Starches are a type of carbohydrate.  So are sugars.

Carbohydrates are a class of biological compounds that have C, H, and O in their structure, and have a ratio of 2 hydrogens for each oxygen.  Then, within the class of carbohydrates we have sugars, and starches ( and cellulose).  So, in your question, starch IS a carbohydrate, but it has hundreds, if not thousands of sugar molecules (glucose) joined together.  The other carbohydrates generally have only a few sugars joined.  So, it would be quite easy to convert starch to glucose, because each molecule of starch has lots, and lots of molecules of glucose.