There is no intrinsic difference between a well-prepared summary and an abstract. Indeed, the Oxford English Dictionary defines abstract as "a summary or epitome of a statement or document," and a summary of a statement is one "containing or comprising the chief points or the sum and substance of a matter." Thus if an editor asks you to prepare a summary, or an abstract, or a synopsis-abstract, you should in each case arrive at the same product.
Of course, the abstract and the summary have different uses. Separate publication of abstracts provides a means of rapidly distributing information to large audiences, and this type of publication has become increasingly popular and important in the past 20 years. Although abstracting services often write abstracts, it is probably better to have the author do so himself.
During the past decade more and more journals have adopted the practice of publishing a summary