This is an excellent video describing the differences between forms of government. Understanding the historical roots of western governance is important when forming opinions about current events.
One minor criticism I have for the video is the absence of constitutional monarchy as a form. This is the one in the Commonwealth countries (England, Canada, Australia, etc.). However, as it mirrors a Republic with the main difference being the head of state (a monarch and president respectively) the description of a Republic also applies to those countries.