Government is one of the most fundamental elements of your interstellar domain. Choosing a form of government is a strategic, long-term consideration, introduced in Master of Orion II.
There are four basic forms of government, each of which has an advanced government form available. You cannot change your government, except for upgrading to a superior form.
There are multiple government forms available. You can switch between them in the game, though it will cause significant unrest throughout your empire. They are shown here ordered (roughly) according to the structure of power, from one concentrated in the hands of a single entity to power being absolutely distributed.