A synopsis states your main line of argument, and depending on the discipline (compare the Political Science and History examples below), alludes to the essential reasons for adopting that line of argument and your main conclusion/s. Usually this kind of synopsis is a stand-alone item, on a separate page from the main text.
Synopsis - Sample from Political Science
In this essay it will be argued that the blame for the political and constitutional crisis of 1975 can be shared by the Governor-General, the Prime Minister and leader of the Opposition, two non-Labor Premiers and the Chief Justice of the High Court and, as conventions were broken by all concerned, some problems have been revealed in the Constitution.
Synopsis - Sample from History
A turning point during the period 70BC to 68AD was when Pompey died and Julius Caesar received his Dictatorship for life. Throughout the late Republican period, powerful men such as Pompey, Crassus, Cicero, Clodius, Cato and Caesar gained power through wealth, religion, military achievements, legal and oratorical means, political appointments and offices, lineage, marriage and through gaining the support of a wide range of people. When Caesar became life-Dictator, although the basis of power did not change, the locus did. Caesar no longer shared or relied on other powerful figures to increase his power. Caesar was not in power long enough to make a significant difference to Rome, but he paved the way for Augustus to hold the office of Princeps, therefore setting the scene for change.
There are two other kinds of synopsis. In literature, a synopsis of a novel summarises the plot. Similarly, in a book review, a synopsis of what the book covers is required at the beginning.
Sometimes, in disciplines such as History, you may be required to write a synoptic essay. This is usually an overview of your understanding of the major themes studied in the unit/subject (and within the discipline context), often indicating how your understanding of the themes has developed over time. In the context of History there is often no need to include references/footnotes in a synoptic essay, but check this with your tutor/lecturer.