Your question is somehow very unspecific.
Generally there are three forms of Amount of Rulers: One, Some, All citizens and the differentiating them by Altruistic(good) and Egoistic(bad) introduced by Aristotle.
This creates a table of the Form:
Altruistic | Egoistic
One: (Philosopher)King | Tyranis
Some: Aristocracy |Oligarchy
All: Polity/Democracy | ...
I'll try to answer your last point re why there has been no communist society and, hopefully, cover some of your other points on the way.
Firstly, I'm going to focus on a Marxist approach as that's been the basis for most non-trivial attempts at forming a communist society.
So, what is communism? From a Marxist standpoint it is a society that follows the ...
A dictatorship is a society ruled over by a dictator. Democracy has been used to refer to various different political systems such as the voting system in ancient Athens, the first past the post voting system in the UK, and proportional representation in other countries such as Israel.
Communism is a system in which property (or just capital) is owned by ...
Your question is rather simple and general at one level, but in historical specificity complex, never fully definable, and rife with paradoxes, such as the "dictatorship of the proletariate." I see that the mention of communism has elicited the usual misleading "gulag" rants, so I offer another point of view.
Dictatorship may be thought ...
I've read a few of the answers and have to agree that it can be a complex issue.
But, I take note of your question. You've asked someone to highlight the main differences between the three. That means avoiding all the various in-depth explanations that cut across political science, sociology, and economics.
However, before I give you my ...
In an attempt to give you bare conceptual definitions:
Dictatorship : a form of governance in which absolute power is exercised by a sole authority
Democracy : a form of governance in which power is divided to the demos (the people)
Communism : a form of organization of an oikos in which one privileges the production of value that is shared in "common" (...