This is a very good question aswathy123 because they are similar in many ways, but very different in others. When used in a business environment, both their function and definitions are different and a good place to start is with function.
By function, an abstract is the opening page -- normally after the cover page but before a Table of Contents -- of a study, plan or other business-use document that has a conclusion and asks the reader for a decision in most cases. The abstract is usually no more than one paragraph of five to seven carefully developed sentences that provide the reader with what the paper is about, what the document intends to do, what methodology was used, what it will conclude and what action is desired from the reader. When writing collegiate papers of four or more pages -- e.g. APA, MLA, Harvard format (double-spaced) -- an abstract is usually required. However, having an abstract in shorter papers can be a benefit by providing the reader with information on what it is they are about to read and the logic of why they need to read it. So by definition, the abstract provides what the paper is about, what it intends to discuss, the methodology used to develop conclusions and recommendations and to ask the reader to take some action.
The synopsis on the other hand, is less formal, can be longer as needed and usually provides the overall content of a paper, document, article or other business document or activity. It does not necessarily need to have an end result such as a conclusion or a decision. It is usually used to provide a quick snapshot of a more complex paper or activity and provides the reader with the general content, findings and results. Also, a synopsis may not have the original piece it is providing a summary of attached or even available, whereas an abstract always is a lead-in to the paper it summarizes